Remove touch scroll chaining from main thread scrolling.

Source/platform/PlatformGestureEvent.h

Index: Source/platform/PlatformGestureEvent.h
diff --git a/Source/platform/PlatformGestureEvent.h b/Source/platform/PlatformGestureEvent.h
index a1d8be5e0506953525615d3b62812b9567fb318b..ab7ea9e555ea8147e53fcfdbf0c297cd5e73ebc4 100644
--- a/Source/platform/PlatformGestureEvent.h
+++ b/Source/platform/PlatformGestureEvent.h
@@ -122,8 +122,11 @@ public:
 
     bool preventPropagation() const
     {
+        // TODO(tdresser) Once we've decided if we're getting rid of scroll
+        // chaining, we should remove all scroll chaining related logic. See
+        // crbug.com/526462 for details.
         ASSERT(m_type == PlatformEvent::GestureScrollUpdate);
-        return m_data.m_scroll.m_preventPropagation;
+        return true;
     }
 
     float scale() const