Scroll on main if impl-hit testing isn't guaranteed to be correct

cc/input/input_handler.h

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_INPUT_INPUT_HANDLER_H_
 #define CC_INPUT_INPUT_HANDLER_H_
 
 #include "base/basictypes.h"
 #include "base/time/time.h"
 #include "cc/base/cc_export.h"
@@ skipping 33 matching lines @@
  private:
   DISALLOW_COPY_AND_ASSIGN(InputHandlerClient);
 };
 
 // The InputHandler is a way for the embedders to interact with the impl thread
 // side of the compositor implementation. There is one InputHandler per
 // LayerTreeHost. To use the input handler, implement the InputHanderClient
 // interface and bind it to the handler on the compositor thread.
 class CC_EXPORT InputHandler {
  public:
-  enum ScrollStatus { ScrollOnMainThread, ScrollStarted, ScrollIgnored };
+  // Note these are used in a histogram. Do not reorder or delete existing
+  // entries.
+  enum ScrollStatus {
+    ScrollOnMainThread = 0,
+    ScrollStarted,
+    ScrollIgnored,
+    ScrollUnknown,
+    // This must be the last entry.
+    ScrollStatusCount
+  };
   enum ScrollInputType { Gesture, Wheel, NonBubblingGesture };
 
   // 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.
   virtual void BindToClient(InputHandlerClient* client) = 0;
 
   // Selects a layer to be scrolled at a given point in viewport (logical
   // pixel) coordinates. Returns ScrollStarted if the layer at the coordinates
   // can be scrolled, ScrollOnMainThread if the scroll event should instead be
@@ skipping 67 matching lines @@
   InputHandler() {}
   virtual ~InputHandler() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(InputHandler);
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_INPUT_HANDLER_H_