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..099f44fdea6f3f7b8f4098fd9f6723abeb3e22f8 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -1006,11 +1006,24 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Ash.WindowSelector.TimeBetweenActiveWindowChanges"
+    units="seconds">

[Mark P, 2015/05/15 19:26:05]

Given that this histogram recording goes through a similar code path as the one
below, I'm surprised they have different units.  Is that correct?

[tdanderson, 2015/05/15 19:50:51]

Both histograms should be in seconds in my opinion, but
I'm hesitant to change the units of TimeBetweenUse since
I imagine this would complicate trying to interpret the
metrics over time.

TimeBetweenActiveWindowChanges is in a family of histograms
bruthig@ and I have been adding, which goes through a 
newly-added centralized location for task-switching metrics
(TaskSwitchMetricsRecorder). The TimeBetweenUse histogram
has been around for a lot longer and its recording
actually lives directly in the overview mode code.

[Mark P, 2015/05/15 19:56:26]

Oh, as long as the code is different then this is okay with me.
I was more concerned that the code does the same measurement
and one of these units= fields in histograms.xml is wrong.  If
they're right, everything is fine.

FWIW, if you want to change how code emits to a histogram
(e..g, what units that histogram is in), you should change
the histogram name at the same time.

+  <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. Not
+    recorded on the first time overview mode is entered after startup.

[Mark P, 2015/05/15 19:26:05]

instead of
Not recorded on the first time overview mode is entered after startup.
perhaps
Only recorded on the second and later times that the user selected a window
which was not previously active.

I believe this is more correct, isn't it?

[tdanderson, 2015/05/15 19:50:51]

Yes, I like that wording better. I've updated it for both histogram
descriptions.

+  </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. Not recorded on the first time overview mode is entered
+    after startup.
   </summary>
 </histogram>