Web Animations: Don't participate in grouping when start time is set

Source/core/animation/CompositorPendingAnimations.cpp

Index: Source/core/animation/CompositorPendingAnimations.cpp
diff --git a/Source/core/animation/CompositorPendingAnimations.cpp b/Source/core/animation/CompositorPendingAnimations.cpp
index 06c0fd0c8c23e5c481deb94c3979575afdd2a711..28ee306a8579feb2980ecd42c562e05bc598b356 100644
--- a/Source/core/animation/CompositorPendingAnimations.cpp
+++ b/Source/core/animation/CompositorPendingAnimations.cpp
@@ -65,14 +65,17 @@ bool CompositorPendingAnimations::update(bool startOnCompositor)
     HeapVector<Member<Animation>> deferred;
     animations.swap(m_pending);
     int compositorGroup = ++m_compositorGroup;
-    if (compositorGroup == 0) {
-        // Wrap around, skipping 0.
+    while (compositorGroup == 0 || compositorGroup == 1) {

[alancutter (OOO until 2018), 2015/09/16 05:35:30]

Nit: We should use an enum instead of magic numbers.

+        // Wrap around, skipping 0, 1.
+        // * 0 is reserved for automatic assignment
+        // * 1 is used for animations with a specified start time
         compositorGroup = ++m_compositorGroup;
     }
 
     for (auto& animation : animations) {
         bool hadCompositorAnimation = animation->hasActiveAnimationsOnCompositor();
-        if (animation->preCommit(compositorGroup, startOnCompositor)) {
+        // Animations with a start time do not participate in compositor start-time grouping.
+        if (animation->preCommit(animation->hasStartTime() ? 1 : compositorGroup, startOnCompositor)) {
             if (animation->hasActiveAnimationsOnCompositor() && !hadCompositorAnimation) {
                 startedSynchronizedOnCompositor = true;
             }