Coalesce async touch move events until the ack back from render

content/common/input_messages.h

Index: content/common/input_messages.h
diff --git a/content/common/input_messages.h b/content/common/input_messages.h
index 80d30430dedaccc5c8970172e8588afeb17b8dd7..4a4f9b8095e84a9f1ccb93db6783b3d2bf222bf3 100644
--- a/content/common/input_messages.h
+++ b/content/common/input_messages.h
@@ -6,6 +6,7 @@
 // order relative to input events.
 // Multiply-included message file, hence no include guard.
 
+#include "base/basictypes.h"
 #include "base/strings/string16.h"
 #include "content/common/content_export.h"
 #include "content/common/content_param_traits.h"
@@ -112,6 +113,7 @@ IPC_STRUCT_BEGIN(InputHostMsg_HandleInputEvent_ACK_Params)
   IPC_STRUCT_MEMBER(ui::LatencyInfo, latency)
   // TODO(jdduke): Use Optional<T> type to avoid heap alloc, crbug.com/375002.
   IPC_STRUCT_MEMBER(scoped_ptr<content::DidOverscrollParams>, overscroll)
+  IPC_STRUCT_MEMBER(uint32, unique_touch_event_id)
 IPC_STRUCT_END()
 
 // Sends an input event to the render widget.