Log the amount of time that the RecentTabsPage is in the foreground to UMA

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 600c054a63fd5257aa6a4aa674072bd1dddb3e66..600c70aeb0dd45887b0e30448a08bc3d2f661534 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -32684,6 +32684,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="RecentsTabsPage.TimeVisibleAndroid" units="milliseconds">

[Ilya Sherman, 2015/06/01 21:46:43]

It looks like you're introducing a new group name, "RecentTabsPage", into the
top-level namespace.  That might be appropriate; but before you do, please
double-check whether there is any existing group that is appropriate.  If not,
please think about whether there's a way to generalize the new group name so
that it's likely to carve out a reasonably sized portion of the top-level
namespace.

[pkotwicz, 2015/06/02 15:35:43]

I think the new namespace makes sense. The "Recent Tabs" page is  a top level
page. We also have a top level namespace for the "New Tab" page (NewTabPage.*)
and the "History" page (WebHistory.*)

[Ilya Sherman, 2015/06/02 19:10:51]

Are you planning to introduce several additional metrics for this page?  If so,
then it's probably reasonable to have a top-level group.  But if this is
essentially a one-off metric, then I think it would be best to think more about
whether it makes sense to fit into another group.  FWIW,
"NewTabPage.RecentTabsPage.TimeVisibleAndroid" is not unreasonably long,
compared to other histogram names.  The dashboard provides typeahead
autocomplete, so it's not like you'll need to be typing the whole thing out.

+  <owner>pkotwicz@chromium.org</owner>
+  <summary>
+    Records the amount of time that the &quot;Recent Tabs&quot; page is visible
+    and Chrome is in the foreground on Android.

[Ilya Sherman, 2015/06/01 21:46:43]

When is this recorded -- when the "Recent Tabs" page is closed or Chrome is
backgrounded?

[pkotwicz, 2015/06/02 15:35:43]

I have updated the comment to be clearer. The metric is recorded whenever the
page moves out of the foreground. This includes when the page is closed, when
the user switches to a different tab and when Chrome is backgrounded.

+  </summary>
+</histogram>
+
 <histogram name="RecoveryComponent.Event" enum="RecoveryComponentEvent">
   <owner>robertshield@chromium.org</owner>
   <summary>Log each stage of a recovery component event.</summary>