Bug 1181392 part 3 - Remove use of IsFinishedTransition in KeyframeEffectReadOnly; r=dbaron

KeyframeEffectReadOnly uses IsFinishedTransition to exclude finished transitions
from certain tests. This check, however, is redundant in each case.
This is because any effect marked as IsFinishedTransition will have the
following properties:
- owning animation's PlayState() == Finished or Idle
- animation phase = after or null
- progress = null (this is because transitions don't fill forwards)

dom/animation/KeyframeEffect.cpp

@@ -236,8 +236,7 @@ KeyframeEffectReadOnly::ActiveDuration(const AnimationTiming& aTiming)
 bool
 KeyframeEffectReadOnly::IsInPlay(const Animation& aAnimation) const
 {
-  if (IsFinishedTransition() ||
-      aAnimation.PlayState() == AnimationPlayState::Finished) {
+  if (aAnimation.PlayState() == AnimationPlayState::Finished) {
     return false;
   }
 
@@ -248,8 +247,7 @@ KeyframeEffectReadOnly::IsInPlay(const Animation& aAnimation) const
 bool
 KeyframeEffectReadOnly::IsCurrent(const Animation& aAnimation) const
 {
-  if (IsFinishedTransition() ||
-      aAnimation.PlayState() == AnimationPlayState::Finished) {
+  if (aAnimation.PlayState() == AnimationPlayState::Finished) {
     return false;
   }
 
@@ -261,10 +259,6 @@ KeyframeEffectReadOnly::IsCurrent(const Animation& aAnimation) const
 bool
 KeyframeEffectReadOnly::IsInEffect() const
 {
-  if (IsFinishedTransition()) {
-    return false;
-  }
-
   ComputedTiming computedTiming = GetComputedTiming();
   return computedTiming.mProgress != ComputedTiming::kNullProgress;
 }