Adds UMA for screen rotation animation smoothness.

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 878604d99644e15ecdff50b057639a60701dcc83..e8b4641f524b461263cd0e72abc157a16c41e849 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -1877,6 +1877,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Ash.Rotation.AnimationSmoothness" units="%">
+  <owner>wutao@chromium.org</owner>
+  <summary>
+    Relative smoothness of animations when rotating screen. 100% represents
+    ideally smooth 60 frames per second.

[Mark P, 2017/03/24 20:32:41]

Please state when this is recorded.
https://chromium.googlesource.com/chromium/src.git/+/HEAD/tools/metrics/histo...

Also, what does 0% or 50% represent.  Is 50%, say, smooth at 30 frames per
second, or half the frames are unsmooth, or half the pixels are unsmooth, or ...

[wutao, 2017/03/24 21:48:24]

Added when it is recorded and added more descriptions.

+  </summary>
+</histogram>
+
 <histogram name="Ash.Shelf.Menu.NumItemsEnabledUponSelection" units="Count">
   <owner>bruthig@google.com</owner>
   <owner>tdanderson@google.com</owner>