This would make things consistent with how -fprofile-instr-use is deployed in WebCore.xcconfig and JavaScriptCore.xcconfig. This was added to BaseTarget.xcconfig in the first place, since adding it to WebKit.xcconfig breaks the build, due to CPP flags no longer including the `-D__STDC_WANT_LIB_EXT1__=1` option. See #238119 for more details.
<rdar://problem/91177830>