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 4cd6fc81fc082bab27945e135322d81cb872a5d2..d4e6528addf75a71a04269d9556ebcaf402b3e39 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -32741,6 +32741,18 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="RecentTabsPage.TimeVisibleAndroid" units="milliseconds">

[nyquist, 2015/06/02 16:36:03]

Given Recent Tabs is a subpage, and part of NewTabPage, it could be argued that
it would be OK to just use:
NewTabPage.RecentTabsPageTimeVisibleAndroid

I don't feel strongly about this though.

[pkotwicz, 2015/06/02 17:13:51]

NewTabPage.RecentTabsPageTimeVisibleAndroid is too long for my liking

+  <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. The metric is recorded whenever
+    the &quot;Recent Tabs&quot; page moves out of the foreground. The metric is
+    recorded when the &quot;Recent Tabs&quot; page is closed, when the user
+    switches from the &quot;Recent Tabs&quot; page 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>