Bug 287793

We fail the "Playback rate affects whether active phase boundary is inclusive" subtest in `scroll-animations/scroll-timelines/scroll-animation-effect-phases.tentative.html`. There are two issues here. The first issue is a faulty assertion function where `assert_phase` does not deal with `currentTime = 100%` being in the "active" phase for animations associated with a progress-based timeline. After fixing this we hit a debug ASSERT in WebKit though. Under `setBindingsTimeline()` at the very end of this test, when we change to the document timeline, we end up in a bad state where after calling `setCurrentTime()` the hold time is 0s but the start time is 100%. After that we set the hold time to the unresolved value since the start time is resolved. Then we're in a state where all the timing values are time-based except for the start time which remains progress-based. The test attempts to resolve this by setting `animation.currentTime = 0` right after, but we hit the debug assertion before being able to set that value.