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..8a1e807a94511e7616efc317547ba5a853c4d6af 100644
--- a/public/web/WebInputEvent.h
+++ b/public/web/WebInputEvent.h
@@ -520,11 +520,17 @@ public:
     // generated.
     bool causesScrollingIfUncanceled;
 
-    WebTouchEvent()
+    // A unique identifier for the touch event.
+    uint64_t uniqueTouchEventId;
+
+    // Pass the uniqueTouchEventId to the WebTouchEvent's constructor to
+    // guarantee that every object has a valid touch Id.
+    WebTouchEvent(uint64_t uniqueTouchId)
         : WebInputEvent(sizeof(WebTouchEvent))
         , touchesLength(0)
         , cancelable(true)
         , causesScrollingIfUncanceled(false)
+        , uniqueTouchEventId(uniqueTouchId)
     {
     }
 };