Support compositing for active animations in SPv2.

third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp

Index: third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
index dc65ccae69b4a1fd97fe66e7016d7fd23657e8a3..0d6eb3ff755e254fb36ccacd927a6f984d5d35f5 100644
--- a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
@@ -319,6 +319,10 @@ static CompositingReasons compositingReasonsForTransform(
   if (CompositingReasonFinder::requiresCompositingForTransform(object))
     compositingReasons |= CompositingReason3DTransform;
 
+  if (CompositingReasonFinder::requiresCompositingForAnimation(
+          object.styleRef()))
+    compositingReasons |= CompositingReasonActiveAnimation;
+
   if (object.styleRef().hasWillChangeCompositingHint() &&
       !object.styleRef().subtreeWillChangeContents())
     compositingReasons |= CompositingReasonWillChangeCompositingHint;