The test was running fine but tarted to become flaky since:

r258134                       NOERROR
[r258135-r258138]             UNKNOWN
r258139                       TEXT PASS (Expected: PASS)
[r258140-r258144]             UNKNOWN
[r258145-r258146]             NOERROR

Within the range possible cases might be r258129 or r258131.

Diff:

See: https://build.webkit.org/results/GTK%20Linux%2064-bit%20Release%20(Tests)/r258139%20(12911)/storage/domstorage/events/basic-setattribute-diff.txt

--- /home/slave/webkitgtk/gtk-linux-64-release-tests/build/layout-test-results/storage/domstorage/events/basic-setattribute-expected.txt
+++ /home/slave/webkitgtk/gtk-linux-64-release-tests/build/layout-test-results/storage/domstorage/events/basic-setattribute-actual.txt
@@ -65,29 +65,32 @@
 storage.setItem('a', '1')
 storage.setItem('b', '2')
 storage.setItem('b', '3')
-PASS storageEventList.length is 5
-PASS storageEventList[1].key is "FU"
-PASS storageEventList[1].oldValue is null
-PASS storageEventList[1].newValue is "BAR"
-PASS storageEventList[2].key is "a"
-PASS storageEventList[2].oldValue is null
-PASS storageEventList[2].newValue is "1"
-PASS storageEventList[3].key is "b"
-PASS storageEventList[3].oldValue is null
-PASS storageEventList[3].newValue is "2"
-PASS storageEventList[4].key is "b"
-PASS storageEventList[4].oldValue is "2"
-PASS storageEventList[4].newValue is "3"
+FAIL Timeout: only got 1, expected 5 events
+FAIL storageEventList.length should be 5. Was 1.
+FAIL storageEventList[1].key should be FU. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[1].key')
+FAIL storageEventList[1].oldValue should be null. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[1].oldValue')
+FAIL storageEventList[1].newValue should be BAR. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[1].newValue')
+FAIL storageEventList[2].key should be a. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[2].key')
+FAIL storageEventList[2].oldValue should be null. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[2].oldValue')
+FAIL storageEventList[2].newValue should be 1. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[2].newValue')
+FAIL storageEventList[3].key should be b. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[3].key')
+FAIL storageEventList[3].oldValue should be null. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[3].oldValue')
+FAIL storageEventList[3].newValue should be 2. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[3].newValue')
+FAIL storageEventList[4].key should be b. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[4].key')
+FAIL storageEventList[4].oldValue should be 2. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[4].oldValue')
+FAIL storageEventList[4].newValue should be 3. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[4].newValue')
 storage.removeItem('FOO')
-PASS storageEventList.length is 6
-PASS storageEventList[5].key is "FOO"
-PASS storageEventList[5].oldValue is "BAR"
-PASS storageEventList[5].newValue is null
+FAIL Timeout: only got 1, expected 6 events
+FAIL storageEventList.length should be 6. Was 1.
+FAIL storageEventList[5].key should be FOO. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[5].key')
+FAIL storageEventList[5].oldValue should be BAR. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[5].oldValue')
+FAIL storageEventList[5].newValue should be null. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[5].newValue')
 storage.removeItem('FU')
-PASS storageEventList.length is 7
-PASS storageEventList[6].key is "FU"
-PASS storageEventList[6].oldValue is "BAR"
-PASS storageEventList[6].newValue is null
+FAIL Timeout: only got 1, expected 7 events
+FAIL storageEventList.length should be 7. Was 1.
+FAIL storageEventList[6].key should be FU. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[6].key')
+FAIL storageEventList[6].oldValue should be BAR. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[6].oldValue')
+FAIL storageEventList[6].newValue should be null. Threw exception TypeError: undefined is not an object (evaluating 'storageEventList[6].newValue')
 storage.clear()
 PASS storageEventList.length is 8
 PASS storageEventList[7].key is null