mac: Add a metric for the time blocked by WaitForSurface().

content/browser/renderer_host/render_widget_host_impl.cc

Index: content/browser/renderer_host/render_widget_host_impl.cc
diff --git a/content/browser/renderer_host/render_widget_host_impl.cc b/content/browser/renderer_host/render_widget_host_impl.cc
index 986bda4ecce621a7506bd15622cf455f13b29864..001f5b7e35aac56d03bad82eeb6bb0e4af5ee8c7 100644
--- a/content/browser/renderer_host/render_widget_host_impl.cc
+++ b/content/browser/renderer_host/render_widget_host_impl.cc
@@ -805,7 +805,8 @@ void RenderWidgetHostImpl::WaitForSurface() {
   }
 
   TimeDelta max_delay = TimeDelta::FromMilliseconds(kPaintMsgTimeoutMS);
-  TimeTicks end_time = TimeTicks::Now() + max_delay;
+  TimeTicks start_time = TimeTicks::Now();
+  TimeTicks end_time = start_time + max_delay;
   do {
     TRACE_EVENT0("renderer_host", "WaitForSurface::WaitForUpdate");
 
@@ -835,6 +836,11 @@ void RenderWidgetHostImpl::WaitForSurface() {
     // BackingStore messages to get to the latest.
     max_delay = end_time - TimeTicks::Now();
   } while (max_delay > TimeDelta::FromSeconds(0));
+

[ccameron, 2015/01/06 02:14:21]

It should be possible to combine the base::TimeTicks::Now() from lines 837 and
840 (maybe have start_time, current_time, end_time). Not a huge difference,
though.

[erikchen, 2015/01/06 03:23:16]

It saves a call to TimeTicks::Now(), plus the original code was structured oddly
anyways. Done.

+  TimeDelta wait_duration = TimeTicks::Now() - start_time;
+  UMA_HISTOGRAM_CUSTOM_TIMES("OSX.RendererHost.SurfaceWaitTime", wait_duration,
+                             TimeDelta::FromMilliseconds(1),
+                             TimeDelta::FromMilliseconds(200), 50);
 }
 #endif