Add unique_event_id to the WebTouchEvent

public/web/WebInputEvent.h

Index: public/web/WebInputEvent.h
diff --git a/public/web/WebInputEvent.h b/public/web/WebInputEvent.h
index 845454f8d36e4c8d11bd6776efd523ba2bc061c5..c4b22c51030898daa6602633b48dd937d02c965d 100644
--- a/public/web/WebInputEvent.h
+++ b/public/web/WebInputEvent.h
@@ -520,11 +520,15 @@ public:
     // 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.

     {
     }
 };