Constrain transform animation direct compositing reason to transforms.

third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
index f414c68cf594e11a019bce909050a55b78c4283b..9587807e9e6e8c90989c9ba6d3b857e07911299e 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
@@ -178,6 +178,13 @@ bool CompositingReasonFinder::requiresCompositingForAnimation(
   return style.shouldCompositeForCurrentAnimations();
 }
 
+bool CompositingReasonFinder::requiresCompositingForTransformAnimation(
+    const ComputedStyle& style) {
+  return style.subtreeWillChangeContents()
+             ? style.isRunningTransformAnimationOnCompositor()
+             : style.hasCurrentTransformAnimation();
+}
+
 bool CompositingReasonFinder::requiresCompositingForScrollDependentPosition(
     const PaintLayer* layer) const {
   if (layer->layoutObject()->style()->position() != FixedPosition &&