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

content/renderer/render_widget.cc

Index: content/renderer/render_widget.cc
diff --git a/content/renderer/render_widget.cc b/content/renderer/render_widget.cc
index 7a19c8ee6cca3a6fd5437fbfc33c2dd0d9b4f8c0..17ce1ec8713fb3816f7454b4c33fcb5f1f1fdcec 100644
--- a/content/renderer/render_widget.cc
+++ b/content/renderer/render_widget.cc
@@ -788,6 +788,10 @@ void RenderWidget::OnViewContextSwapBuffersComplete() {
 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;