Expose scroll customization for touch to JS (behind REF).

Source/core/dom/ElementRareData.cpp

Index: Source/core/dom/ElementRareData.cpp
diff --git a/Source/core/dom/ElementRareData.cpp b/Source/core/dom/ElementRareData.cpp
index 08934494c16d767e836bef33f315ebc46ac14930..fc3b8df364b305c02edc689781c7e1a7a0cfe11f 100644
--- a/Source/core/dom/ElementRareData.cpp
+++ b/Source/core/dom/ElementRareData.cpp
@@ -39,7 +39,7 @@ struct SameSizeAsElementRareData : NodeRareData {
     short indices[2];
     LayoutSize sizeForResizing;
     IntSize scrollOffset;
-    void* pointers[13];
+    void* pointers[15];

[haraken, 2015/06/27 08:09:32]

It's unfortunate that we increase sizeof(ElementRareData) for runtime enabled
features that are currently disabled.

[tdresser, 2015/06/29 20:59:33]

Can you suggest an alternative? You previously mentioned the option of using a
v8 hidden value, but that would require a bunch of logic for interacting with
the v8 types.

We'd need to:
- When receiving the ScrollStateCallback, convert it into a v8::Value.
- When calling the ScrollStateCallback, convert it from a v8::Value.
- Fetch the v8::Object for the current element whenever we get or set a scroll
customization method.

I can give this a shot, but it feels a little bit heavier than would be ideal.

 };
 
 CSSStyleDeclaration& ElementRareData::ensureInlineCSSStyleDeclaration(Element* ownerElement)
@@ -72,6 +72,8 @@ DEFINE_TRACE_AFTER_DISPATCH(ElementRareData)
     visitor->trace(m_generatedAfter);
     visitor->trace(m_generatedFirstLetter);
     visitor->trace(m_backdrop);
+    visitor->trace(m_applyScroll);
+    visitor->trace(m_distributeScroll);
     NodeRareData::traceAfterDispatch(visitor);
 }