Remove GPU.CreateBrowserCompositor metric.

Remove GPU.CreateBrowserCompositor metric, which measures how long it
takes to create an instance of cc::LayerTreeHosts. This time is 0us at
96+%ile, and < 20us in 99%ile. I don't think this metric has much value
at the moment. It was initially added for crbug.com/311259, but was not
particular useful even at that time.

Change-Id: I08d9b77cbc3ad65d1b8685448bcbdcdf312a3e2e
Reviewed-on: https://chromium-review.googlesource.com/1127322Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Robert Kaplow (slow) <rkaplow@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575028}

tools/metrics/histograms/histograms.xml

@@ -32185,6 +32185,9 @@ uploading your change for review.
 </histogram>
 
 <histogram name="GPU.CreateBrowserCompositor" units="microseconds">
+  <obsolete>
+    Deprecated 07/2018: not useful anymore.
+  </obsolete>
   <owner>vmiura@chromium.org</owner>
   <summary>
     The time that the browser process takes to create the compositor from its

ui/compositor/compositor.cc

@@ -195,8 +195,6 @@ Compositor::Compositor(const viz::FrameSinkId& frame_sink_id,
   settings.always_request_presentation_time =
       command_line->HasSwitch(cc::switches::kAlwaysRequestPresentationTime);
 
-  base::TimeTicks before_create = base::TimeTicks::Now();
-
   animation_host_ = cc::AnimationHost::CreateMainInstance();
 
   cc::LayerTreeHost::InitParams params;
@@ -206,8 +204,6 @@ Compositor::Compositor(const viz::FrameSinkId& frame_sink_id,
   params.main_task_runner = task_runner_;
   params.mutator_host = animation_host_.get();
   host_ = cc::LayerTreeHost::CreateSingleThreaded(this, &params);
-  UMA_HISTOGRAM_TIMES("GPU.CreateBrowserCompositor",
-                      base::TimeTicks::Now() - before_create);
 
   if (base::FeatureList::IsEnabled(features::kUiCompositorScrollWithLayers) &&
       host_->GetInputHandler()) {