Constrain transform animation direct compositing reason to transforms.

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

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/layout/compositing/CompositingReasonFinder.h"
 
 #include "core/CSSPropertyNames.h"
 #include "core/dom/Document.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/Settings.h"
@@ skipping 160 matching lines @@
 }
 
 bool CompositingReasonFinder::requiresCompositingForAnimation(
     const ComputedStyle& style) {
   if (style.subtreeWillChangeContents())
     return style.isRunningAnimationOnCompositor();
 
   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 &&
       layer->layoutObject()->style()->position() != StickyPosition)
     return false;
 
   if (!(m_compositingTriggers & ViewportConstrainedPositionedTrigger) &&
       (!RuntimeEnabledFeatures::compositeOpaqueFixedPositionEnabled() ||
        !layer->backgroundIsKnownToBeOpaqueInRect(
            LayoutRect(layer->boundingBoxForCompositing())) ||
        layer->compositesWithTransform() || layer->compositesWithOpacity())) {
     return false;
   }
   // Don't promote fixed position elements that are descendants of a non-view
   // container, e.g. transformed elements.  They will stay fixed wrt the
   // container rather than the enclosing frame.
   if (layer->sticksToViewport())
     return m_layoutView.frameView()->isScrollable();
   return layer->layoutObject()->style()->position() == StickyPosition &&
          layer->ancestorOverflowLayer()->scrollsOverflow();
 }
 
 }  // namespace blink