Add telemetry to track the time an event spent waiting for the main thread.

content/renderer/render_widget.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/render_widget.h"
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
@@ skipping 770 matching lines @@
     return;
   }
 
   // Continue painting if necessary...
   DoDeferredUpdateAndSendInputAck();
 }
 
 void RenderWidget::OnHandleInputEvent(const WebKit::WebInputEvent* input_event,
                                       const ui::LatencyInfo& latency_info,
                                       bool is_keyboard_shortcut) {
+  const_cast<ui::LatencyInfo&>(latency_info).AddLatencyNumber(

[nduca, 2013/08/15 01:41:04]

hmmm remember how we talked about fixing the way input events are tracked wrt
frame generation?

i think the fact that youre doing a const cast here to get rid of the const is a
sign of needing to do that cleanup first. it seems better that we stashing the
input event here, 

once you have that, it'll be easy to come back here and add this call without a
const çast.

+      ui::INPUT_EVENT_LATENCY_EVENT_RECEIVED_ON_MAIN_THREAD_COMPONENT,
+      routing_id(), 0);
+
   handling_input_event_ = true;
   if (!input_event) {
     handling_input_event_ = false;
     return;
   }
 
   const char* const event_name = GetEventName(input_event->type);
   TRACE_EVENT1("renderer", "RenderWidget::OnHandleInputEvent",
                "event", event_name);
 
@@ skipping 1721 matching lines @@
 
   if (!context->InitializeWithDefaultBufferSizes(
           attributes,
           false /* bind generates resources */,
           CAUSE_FOR_GPU_LAUNCH_WEBGRAPHICSCONTEXT3DCOMMANDBUFFERIMPL_INITIALIZE))
     return NULL;
   return context.release();
 }
 
 }  // namespace content