Add UMA metrics for window selection and overview.

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 4d9d75b27516baef06a3a8c593922c40fb88a68a..27e581c1214bf1430df661e46536e0cb29c3ed9f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -254,6 +254,27 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Ash.WindowSelector.Items">
+  <summary>
+    The number of items (single windows or groups of windows such as panels) in
+    the window selection.

[Ilya Sherman, 2013/09/19 23:08:04]

What is "the window selection"?  Could you expand this description a bit to
clarify for folks like myself who are not super familiar with the jargon of this
feature?

[flackr, 2013/09/19 23:23:45]

Done.

+  </summary>
+</histogram>
+
+<histogram name="Ash.WindowSelector.TimeBetweenUse" units="milliseconds">
+  <summary>
+    The amount of time between using overview or alt tab cycling to select

[Ilya Sherman, 2013/09/19 23:08:04]

I think it would be clearer to phrase this along the lines of "The amount of
time between uses of the overview mode to select windows.  The overview mode can
be entered either by alt-tabbing or by pressing the overview button."  The two
key changes are "uses of" in place of "using", and splitting off the description
of the ways to access the feature from the name of the feature, making for a
less complex first sentence.  Feel free to improve the wording further, but I'd
recommend keeping those two properties :)

[flackr, 2013/09/19 23:23:45]

Done.

+    windows.
+  </summary>
+</histogram>
+
+<histogram name="Ash.WindowSelector.TimeInOverview" units="milliseconds">
+  <summary>
+    The amount of time spent in overview mode by either alt tabbing or pressing
+    the overview button.
+  </summary>
+</histogram>
+
 <histogram name="AsyncDNS.AttemptCountFail">
   <summary>
     Count of DnsAttempts before DnsTransaction completes with failure.