Send the pending input event ack before the UpdateRect message

This can potentially cut the mouse move, mouse wheel, and touch move
latency for platforms that send the UpdateRect_ACK message within the
UpdateRect IPC handler. Today, it seems that windows and mac will
sometimes send the UpdateRect_ACK synchronously, while linux never will.

Patch also includes some extra instrumentation to make this easier to
track that we may or may not want to land.

BUG=none


Review URL: http://codereview.chromium.org/8820001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@114141 0039d316-1c4b-4281-b951-d872f2087c98

content/browser/renderer_host/render_widget_host.cc

@@ -554,7 +554,8 @@ void RenderWidgetHost::StopHangMonitorTimeout() {
 }
 
 void RenderWidgetHost::ForwardMouseEvent(const WebMouseEvent& mouse_event) {
-  TRACE_EVENT0("renderer_host", "RenderWidgetHost::ForwardMouseEvent");
+  TRACE_EVENT2("renderer_host", "RenderWidgetHost::ForwardMouseEvent",
+               "x", mouse_event.x, "y", mouse_event.y);
   if (ignore_input_events_ || process_->IgnoreInputEvents())
     return;
 

content/renderer/render_widget.cc

@@ -481,8 +481,11 @@ void RenderWidget::OnHandleInputEvent(const IPC::Message& message) {
 
   bool prevent_default = false;
   if (WebInputEvent::isMouseEventType(input_event->type)) {
-    prevent_default = WillHandleMouseEvent(
-        *(static_cast<const WebMouseEvent*>(input_event)));
+    const WebMouseEvent& mouse_event =
+        *static_cast<const WebMouseEvent*>(input_event);
+    TRACE_EVENT2("renderer", "HandleMouseMove",
+                 "x", mouse_event.x, "y", mouse_event.y);
+    prevent_default = WillHandleMouseEvent(mouse_event);
   }
 
   bool processed = prevent_default;
@@ -879,6 +882,13 @@ void RenderWidget::DoDeferredUpdate() {
     webwidget_->composite(false);
   }
 
+  // If we're holding a pending input event ACK, send the ACK before sending the
+  // UpdateReply message so we can receive another input event before the
+  // UpdateRect_ACK on platforms where the UpdateRect_ACK is sent from within
+  // the UpdateRect IPC message handler.
+  if (pending_input_event_ack_.get())
+    Send(pending_input_event_ack_.release());
+
   // If composite() called SwapBuffers, pending_update_params_ will be reset (in
   // OnSwapBuffersPosted), meaning a message has been added to the
   // updates_pending_swap_ queue, that will be sent later. Otherwise, we send