Make 'will-change: contents' suppress compositing in subtree

Source/core/rendering/compositing/CompositingReasonFinder.cpp

Index: Source/core/rendering/compositing/CompositingReasonFinder.cpp
diff --git a/Source/core/rendering/compositing/CompositingReasonFinder.cpp b/Source/core/rendering/compositing/CompositingReasonFinder.cpp
index 1137e0552d5c544956268ee86e7f0709b2ac64a4..6b5b23c90b8fc622cd12ddaabdb17f27c9ec549e 100644
--- a/Source/core/rendering/compositing/CompositingReasonFinder.cpp
+++ b/Source/core/rendering/compositing/CompositingReasonFinder.cpp
@@ -141,6 +141,9 @@ bool CompositingReasonFinder::requiresCompositingForFilters(RenderObject* render
 
 bool CompositingReasonFinder::requiresCompositingForWillChangeCompositingHint(const RenderObject* renderer) const
 {
+    if (renderer->style()->subtreeWillChangeContents())
+        return false;
+
     return renderer->style()->hasWillChangeCompositingHint();
 }
 
@@ -174,6 +177,9 @@ CompositingReasons CompositingReasonFinder::nonStyleDeterminedDirectReasons(cons
 
 bool CompositingReasonFinder::requiresCompositingForAnimation(RenderObject* renderer) const
 {
+    if (renderer->style()->subtreeWillChangeContents())
+        return renderer->style()->isRunningAnimationOnCompositor();
+
     return renderer->style()->shouldCompositeForCurrentAnimations();
 }