Along with https://codereview.chromium.org/23458035, this

adds and makes use of Renderer.AcceleratedFixedRootBackground UMA histogram.

Records three counts:
1. The total number of main frame scrolls.
2. The total number of main frame scrolls with an accelerated fixed root background.
3. The total number of main frame scrolls with an unaccelerated fixed root background.

R=hartmanng@chromium.org,asvitkine@chromium.org
BUG=None

Review URL: https://chromiumcodereview.appspot.com/23983017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@222086 0039d316-1c4b-4281-b951-d872f2087c98

tools/metrics/histograms/histograms.xml

@@ -12753,6 +12753,15 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Renderer.AcceleratedFixedRootBackground"
+    enum="AcceleratedFixedRootBackground">
+  <summary>
+    Keeps track of the number of main frame scrolls with an accelerated fixed
+    root background, the number of main frame scrolls with an unaccelerated
+    fixed root background, and the total number of main frame scrolls.
+  </summary>
+</histogram>
+
 <histogram name="Renderer.CompositedScrolling" enum="CompositedScrolling">
   <summary>
     Total count of the number of RenderLayers which are scrollable areas, need
@@ -18103,6 +18112,12 @@ other types of suffix sets.
   <int value="1" label="Abandoned"/>
 </enum>
 
+<enum name="AcceleratedFixedRootBackground" type="int">
+  <int value="0" label="ScrolledMainFrame"/>
+  <int value="1" label="ScrolledMainFrameWithAcceleratedFixedRootBackground"/>
+  <int value="2" label="ScrolledMainFrameWithUnacceleratedFixedRootBackground"/>
+</enum>
+
 <enum name="AndroidEvictionReason" type="int">
   <int value="0" label="TabUnusedTooLong"/>
   <int value="1" label="TabUnusedInSession"/>