Add unique_event_id to the WebTouchEvent

public/web/WebInputEvent.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 502 matching lines @@
     // must wait for an ACK for this event. If false then no ACK IPC is expected.
     bool cancelable;
 
     // Whether the event will produce scroll-inducing events if uncanceled. This
     // will be true for touchmove events after the platform slop region has been
     // exceeded and fling-generating touchend events. Note that this doesn't
     // necessarily mean content will scroll, only that scroll events will be
     // generated.
     bool causesScrollingIfUncanceled;
 
+    // A unique identifier for the touch event.
+    uint64_t uniqueTouchEventId;
+
     WebTouchEvent()
         : WebInputEvent(sizeof(WebTouchEvent))
         , touchesLength(0)
         , cancelable(true)
         , causesScrollingIfUncanceled(false)
+        , uniqueTouchEventId(1)

[tdresser, 2015/03/16 19:09:55]

We want to ensure that every WebTouchEvent has a uniqueTouchEventId assigned. A
good way to do this would be to default to 0, and assert that the value isn't 0
once we're sure a uniqueTouchEventId should have been assigned.

     {
     }
 };
 
 #pragma pack(pop)
 
 } // namespace blink
 
 #endif