[scheduler] Adds histogram to record the number of frame visibility change.

We want to plumb frame visibility change to GRC. Before that, we want to
understand the volume of IPC. This patch adds histogram to record the frame
visibility when it's changed.

BUG=790680

Change-Id: Ib798308bf18473c1432cbe651ac9217714115b9a
Reviewed-on: https://chromium-review.googlesource.com/807351Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: lpy <lpy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522002}

third_party/WebKit/Source/platform/scheduler/renderer/web_frame_scheduler_impl.cc

@@ -5,6 +5,7 @@
 #include "platform/scheduler/renderer/web_frame_scheduler_impl.h"
 
 #include <memory>
+#include "base/metrics/histogram_macros.h"
 #include "base/trace_event/blame_context.h"
 #include "platform/runtime_enabled_features.h"
 #include "platform/scheduler/base/real_time_domain.h"
@@ -187,6 +188,7 @@ void WebFrameSchedulerImpl::SetFrameVisible(bool frame_visible) {
   DCHECK(parent_web_view_scheduler_);
   if (frame_visible_ == frame_visible)
     return;
+  UMA_HISTOGRAM_BOOLEAN("RendererScheduler.IPC.FrameVisibility", frame_visible);
   bool was_throttled = ShouldThrottleTimers();
   frame_visible_ = frame_visible;
   UpdateThrottling(was_throttled);

tools/metrics/histograms/histograms.xml

@@ -69604,6 +69604,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="RendererScheduler.IPC.FrameVisibility" enum="Boolean">
+  <owner>altimin@chromium.org</owner>
+  <owner>lpy@chromium.org</owner>
+  <summary>
+    This boolean keeps track the count of the visibility change of a frame.
+    Recorded every time when the visibility of a frame is changed.
+  </summary>
+</histogram>
+
 <histogram name="RendererScheduler.MaxQueueingTime" units="ms">
   <owner>maxlg@chromium.org</owner>
   <summary>