Add UMA for CrossFadeAnimation.

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 c309a7f5982044ec4d2c40b0c9ff24b8f1c21a95..e70da41d215f65f841650b94354adb762e6dc669 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -2371,6 +2371,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>The wallpaper type. Recorded at user login.</summary>
 </histogram>
 
+<histogram name="Ash.Window.AnimationSmoothness.CrossFade" units="%">
+  <owner>wutao@chromium.org</owner>
+  <summary>
+    Relative smoothness of cross fade animation when setting window bounds. 100%
+    represents ideally smooth 60 frames per second. 50% represents only 30

[varkha, 2017/05/12 15:14:25]

nit: ...represents when only...

[wutao, 2017/05/12 17:46:37]

Done.

+    frames per second is achieved during the animations. 0% should not happen.
+    This metric is recorded exactly once when WmWindow::SetBoundsDirectCrossFade

[varkha, 2017/05/12 15:14:25]

WmWindow will be going away so maybe just "SetBoundsDirectCrossFade".

[wutao, 2017/05/12 17:46:38]

Done.

+    is called, such as when window is maximized.
+  </summary>
+</histogram>
+
 <histogram name="Ash.WindowCycleController.CycleTime" units="ms">
   <owner>varkha@chromium.org</owner>
   <owner>tbuckley@google.com</owner>