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 daad53c7bd927931e14cb6edae7f2f62659eb14a..48de8d73a99bab8f5d05671a2f1327f7ae8b5f9d 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(

[wkorman, 2016/12/15 23:06:00]

Are we not using CompositingReasonFinder::potentialCompositingReasonsFromStyle()
directly just so that we can be thoughtful and integrate these one at a time?

+          object.styleRef()))
+    compositingReasons |= CompositingReasonActiveAnimation;
+
   if (object.styleRef().hasWillChangeCompositingHint() &&
       !object.styleRef().subtreeWillChangeContents())
     compositingReasons |= CompositingReasonWillChangeCompositingHint;