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

Source/core/dom/ElementRareData.h

Index: Source/core/dom/ElementRareData.h
diff --git a/Source/core/dom/ElementRareData.h b/Source/core/dom/ElementRareData.h
index bcd739cf05d7d796782502e1998914bdb2390157..8f215d553a24564a6f81be8d38bfa5536397128c 100644
--- a/Source/core/dom/ElementRareData.h
+++ b/Source/core/dom/ElementRareData.h
@@ -31,6 +31,7 @@
 #include "core/dom/custom/CustomElementDefinition.h"
 #include "core/dom/shadow/ElementShadow.h"
 #include "core/html/ClassList.h"
+#include "core/page/scrolling/ScrollStateCallback.h"
 #include "core/style/StyleInheritedData.h"
 #include "platform/heap/Handle.h"
 #include "wtf/OwnPtr.h"
@@ -121,6 +122,14 @@ public:
     void setCustomElementDefinition(PassRefPtrWillBeRawPtr<CustomElementDefinition> definition) { m_customElementDefinition = definition; }
     CustomElementDefinition* customElementDefinition() const { return m_customElementDefinition.get(); }
 
+    void setDistributeScroll(ScrollStateCallback* scrollStateCallback) { m_distributeScroll = scrollStateCallback; }
+
+    ScrollStateCallback* getDistributeScroll() { return m_distributeScroll.get(); }
+
+    void setApplyScroll(ScrollStateCallback* scrollStateCallback) { m_applyScroll = scrollStateCallback; }
+
+    ScrollStateCallback* getApplyScroll() { return m_applyScroll.get(); }
+
     AttrNodeList& ensureAttrNodeList();
     AttrNodeList* attrNodeList() { return m_attrNodeList.get(); }
     void removeAttrNodeList() { m_attrNodeList.clear(); }
@@ -152,6 +161,9 @@ private:
     RefPtrWillBeMember<PseudoElement> m_generatedFirstLetter;
     RefPtrWillBeMember<PseudoElement> m_backdrop;
 
+    PersistentWillBeMember<ScrollStateCallback> m_applyScroll;
+    PersistentWillBeMember<ScrollStateCallback> m_distributeScroll;

[tdresser, 2015/07/06 20:43:01]

We need to find somewhere else to stash these, as adding 2*sizeof(Persistent) to
every ElementRareData is an unacceptable size increase (according to haraken@).

[Rick Byers, 2015/07/07 21:24:40]

Can we instead have a map somewhere (per Document?) from Element to
ScrollStateCallback?  Maybe something kind of like EventHandlerRegistry?

[tdresser, 2015/07/13 17:36:52]

Per Document / Framehost would introduce a fair bit of unnecessary complexity,
as Elements can be moved between Documents and FrameHosts. We'd need to pass the
callbacks back and forth when elements are moved around. A static map isn't
particularly elegant, but it lets us tie the lifetime to the lifetime of the
Element without inflating ElementRareData, and doesn't introduce much additional
complexity.

Does this seem reasonable?

[haraken, 2015/07/13 23:43:07]

A static map is fine. See how CustomElementObserverMap works. We can do a
similar thing here.

https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...

+
     explicit ElementRareData(LayoutObject*);
 };