Set the unique_event_id when converting from TouchEvent to WebTouchEvent

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 5bdda8fb73949625632cc5077e56c2addf983c95..dac55e49666874fc096b9818db58ecb1d90e1b90 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.

[jdduke (slow), 2015/03/13 15:31:59]

So, for some time I've been meaning to make the ack a proper structure, with
different fields for different event types.

i.e., overscroll would be a field for scrollupdate/wheel, id would be a field
for touch events, etc... similar to how WebGestureEvent behaves. Not sure if it
makes sense to do it now, but it would ease the pain of adding event
type-specific ack metadata in the future.

   IPC_STRUCT_MEMBER(scoped_ptr<content::DidOverscrollParams>, overscroll)
+  IPC_STRUCT_MEMBER(uint64, unique_event_id)
 IPC_STRUCT_END()
 
 // Sends an input event to the render widget.