Articulate the cancel behavior in WebMouseWheelEvent.

third_party/WebKit/Source/platform/PlatformWheelEvent.h

Index: third_party/WebKit/Source/platform/PlatformWheelEvent.h
diff --git a/third_party/WebKit/Source/platform/PlatformWheelEvent.h b/third_party/WebKit/Source/platform/PlatformWheelEvent.h
index 66a27a4a7eca2d26c07b3008aebf064e8827ea90..19163dbff69ee3b761dfa0a6887db62cf6d8eb5c 100644
--- a/third_party/WebKit/Source/platform/PlatformWheelEvent.h
+++ b/third_party/WebKit/Source/platform/PlatformWheelEvent.h
@@ -66,6 +66,7 @@ public:
         , m_canScroll(true)
         , m_resendingPluginId(-1)
         , m_railsMode(RailsModeFree)
+        , m_dispatchType(Blocking)
 #if OS(MACOSX)
         , m_phase(PlatformWheelEventPhaseNone)
         , m_momentumPhase(PlatformWheelEventPhaseNone)
@@ -92,7 +93,8 @@ public:
     void setCanScroll(bool b) { m_canScroll = b; }
     int resendingPluginId() const { return m_resendingPluginId; }
     RailsMode getRailsMode() const { return m_railsMode; }
-
+    DispatchType dispatchType() const { return m_dispatchType; }
+    bool cancelable() const { return m_dispatchType == PlatformEvent::Blocking; }
 #if OS(MACOSX)
     PlatformWheelEventPhase phase() const { return m_phase; }
     PlatformWheelEventPhase momentumPhase() const { return m_momentumPhase; }
@@ -112,6 +114,7 @@ protected:
     bool m_canScroll;
     int m_resendingPluginId;
     RailsMode m_railsMode;
+    DispatchType m_dispatchType;
 #if OS(MACOSX)
     PlatformWheelEventPhase m_phase;
     PlatformWheelEventPhase m_momentumPhase;