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

Source/core/css/resolver/StyleBuilderCustom.cpp

Index: Source/core/css/resolver/StyleBuilderCustom.cpp
diff --git a/Source/core/css/resolver/StyleBuilderCustom.cpp b/Source/core/css/resolver/StyleBuilderCustom.cpp
index f2e2356769bafca0dac608ae8c4a8e84bef5ab44..6163567943ed497c7297bdc9e5573c23305b4aaf 100644
--- a/Source/core/css/resolver/StyleBuilderCustom.cpp
+++ b/Source/core/css/resolver/StyleBuilderCustom.cpp
@@ -1081,6 +1081,7 @@ void StyleBuilderFunctions::applyInitialCSSPropertyWillChange(StyleResolverState
     state.style()->setWillChangeContents(false);
     state.style()->setWillChangeScrollPosition(false);
     state.style()->setWillChangeProperties(Vector<CSSPropertyID>());
+    state.style()->setSubtreeWillChangeContents(state.parentStyle()->subtreeWillChangeContents());

[esprehn, 2014/05/23 01:05:58]

I don't think this is correct, we don't go through the initial setter normally.
This is only if you do will-change: initial;

[ajuma, 2014/05/23 14:38:31]

For the normal case, RenderStyle::inheritFrom already does:
  rareInheritedData = inheritParent->rareInheritedData;
so we don't need to add any additional logic there.

(But the logic added above is indeed needed for when you do "will-change:
initial".)

 }
 
 void StyleBuilderFunctions::applyInheritCSSPropertyWillChange(StyleResolverState& state)
@@ -1088,6 +1089,7 @@ void StyleBuilderFunctions::applyInheritCSSPropertyWillChange(StyleResolverState
     state.style()->setWillChangeContents(state.parentStyle()->willChangeContents());
     state.style()->setWillChangeScrollPosition(state.parentStyle()->willChangeScrollPosition());
     state.style()->setWillChangeProperties(state.parentStyle()->willChangeProperties());
+    state.style()->setSubtreeWillChangeContents(state.parentStyle()->subtreeWillChangeContents());
 }
 
 void StyleBuilderFunctions::applyValueCSSPropertyWillChange(StyleResolverState& state, CSSValue* value)
@@ -1111,6 +1113,7 @@ void StyleBuilderFunctions::applyValueCSSPropertyWillChange(StyleResolverState&
     state.style()->setWillChangeContents(willChangeContents);
     state.style()->setWillChangeScrollPosition(willChangeScrollPosition);
     state.style()->setWillChangeProperties(willChangeProperties);
+    state.style()->setSubtreeWillChangeContents(willChangeContents || state.parentStyle()->subtreeWillChangeContents());
 }
 
 void StyleBuilderFunctions::applyInitialCSSPropertyContent(StyleResolverState& state)