[scheduler] Monitor renderer load level.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 75f16fdd33e2dc9286685a4235553e439a553fba..2f9de51a32b374977c38202013d4b19b78be97aa 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -47574,6 +47574,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="RendererScheduler.BackgroundRendererMainThreadLoad" units="%">
+  <owner>altimin@chromium.org</owner>
+  <summary>
+    Renderer main thread load when renderer is backgrounded, i.e. percentage of
+    time spent on running tasks.
+
+    This metric is emitted when the renderer main thread task is completed or
+    renderer is backgrounded or foregrounded, at most once per second per
+    renderer amortized.
+  </summary>
+</histogram>
+
 <histogram name="RendererScheduler.ExpectedTaskQueueingDuration" units="ms">
   <owner>tdresser@chromium.org</owner>
   <summary>
@@ -47583,6 +47595,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="RendererScheduler.ForegroundRendererMainThreadLoad" units="%">
+  <owner>altimin@chromium.org</owner>
+  <summary>
+    Renderer main thread load when renderer is foregrounded, i.e. percentage of
+    time spent on running tasks.
+
+    This metric is emitted when the renderer main thread task is completed or
+    renderer is backgrounded or foregrounded, at most once per second per
+    renderer amortized.
+  </summary>
+</histogram>
+
 <histogram name="RendererScheduler.TaskQueueManager.DelayedTaskLateness"
     units="ms">
   <owner>alexclarke@chromium.org</owner>