Record time between switching the active window from Ash 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 9ced647767173738c3892330482b5c12fde5691f..ad456363a63b85636736e17e679fd4fd7cac36fc 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -1006,11 +1006,22 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Ash.WindowSelector.TimeBetweenActiveWindowChanges"
+    units="seconds">
+  <owner>bruthig@chromium.org</owner>
+  <owner>tdanderson@chromium.org</owner>
+  <summary>
+    The amount of time between endings of overview mode sessions which were
+    caused by the user selecting a window which was not previously active.
+  </summary>
+</histogram>
+
 <histogram name="Ash.WindowSelector.TimeBetweenUse" units="milliseconds">
   <owner>flackr@chromium.org</owner>
   <owner>kuscher@google.com</owner>
   <summary>
-    The amount of time between uses of overview mode to switch between windows.
+    The amount of time between uses of overview mode, recorded when overview
+    mode is entered.

[Mark P, 2015/05/15 17:58:37]

(1) I assume this is a clarification and not a change in functionality?
(2) For further clarification, perhaps you should mention "for the second and
later times" or add "No recorded on the first time overview mode is entered
after startup." (or after profile open, or after this chrome binary is updated
to the version that records this metric, if you keep track of this number
between chrome restarts, or ...)

This clarification should probably also be applied to the histogram description
you're adding as well.

[tdanderson, 2015/05/15 19:06:50]

Yes, it's just a clarification since the two histograms do
track similar (but not identical) things.
Done. 
Done.

   </summary>
 </histogram>