Make touch events uncancelable during fling when they are on the current active scroll layer

cc/input/input_handler.h

Index: cc/input/input_handler.h
diff --git a/cc/input/input_handler.h b/cc/input/input_handler.h
index 9b96db789d1d03c4a5fc6fc655288ffb3ad0f0d1..fd4da696c3f463910ff2e66ba48cd01f90231591 100644
--- a/cc/input/input_handler.h
+++ b/cc/input/input_handler.h
@@ -105,6 +105,8 @@ class CC_EXPORT InputHandler {
     NON_BUBBLING_GESTURE
   };
 
+  enum TouchStartHitResult { HANDLER, SAME_LAYER, DIFFERENT_LAYER };

[tdresser, 2016/11/04 17:15:20]

These names aren't clear to me. Same layer as what? They need to make sense in
the context of: |DoTouchEventsBlockScrollAt| (or we could rename that method).

Perhaps:
DoTouchHandlersBlockScrollAt(...)
NO_HANDLER
HANDLER
HANDLER_ON_FLINGING_LAYER

Maybe get a second opinion on this - I'm not too fond of the naming here.

+
   // Binds a client to this handler to receive notifications. Only one client
   // can be bound to an InputHandler. The client must live at least until the
   // handler calls WillShutdown() on the client.
@@ -190,7 +192,8 @@ class CC_EXPORT InputHandler {
 
   // Whether the page should be given the opportunity to suppress scrolling by
   // consuming touch events that started at |viewport_point|.

[tdresser, 2016/11/04 17:15:20]

Update this comment.

[lanwei, 2016/11/11 18:22:45]

Done.

-  virtual bool DoTouchEventsBlockScrollAt(const gfx::Point& viewport_point) = 0;
+  virtual TouchStartHitResult DoTouchEventsBlockScrollAt(
+      const gfx::Point& viewport_point) = 0;
 
   // Calling CreateLatencyInfoSwapPromiseMonitor() to get a scoped
   // LatencyInfoSwapPromiseMonitor. During the life time of the