[ash-md] Adds UMA depth count for Overview mode

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 48932868a0b3a43b13f7c5939e80ec455b39e470..d5d8bf3e9e29da9ee4c508657a43ada48ded71b5 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -1608,6 +1608,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Ash.WindowSelector.SelectionDepth" units="items">
+  <owner>varkha@chromium.org</owner>
+  <owner>tdanderson@chromium.org</owner>
+  <owner>tbuckley@chromium.org</owner>
+  <summary>
+    The number of items in MRU order before the item selected in overview mode
+    at the moment when an item is selected.

[tdanderson, 2016/06/24 00:17:51]

Suggested rewording for clarity, something like: "When a window is selected in
overview mode, records that window's position in the global MRU ordering. 1
represents the most-recently used window, 2 represents the next most-recently
used window, and so on."

[varkha, 2016/06/24 16:31:39]

Done.

+  </summary>
+</histogram>
+
 <histogram name="Ash.WindowSelector.TextFilteringStringLength"
     units="characters">
   <owner>tdanderson@chromium.org</owner>