[Herb] Add metrics to track button usage

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 c9c910a5e467190bb77d9115328862404ff09a12..a1fb3ad03724d38c82c45a6c37d5e4e35c384727 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -250,6 +250,24 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Android.Activity.ChromeTabbedActivity.StopReason"
+    enum="AndroidActivityStopReason">
+  <owner>dfalcantara@chromium.org</owner>
+  <summary>
+    What caused ChromeTabbedActivity#onStop() to be called, which indicates that
+    Chrome is sent to the background.
+  </summary>
+</histogram>
+
+<histogram name="Android.Activity.ChromeTabbedActivity.SystemBackAction"
+    enum="AndroidActivitySystemBackAction">
+  <owner>dfalcantara@chromium.org</owner>
+  <summary>
+    What happened when the user hit the system back button in
+    ChromeTabbedActivity.
+  </summary>
+</histogram>
+
 <histogram name="Android.ArmFpu" enum="AndroidArmFpu">
   <owner>fdegans@chromium.org</owner>
   <summary>
@@ -57887,6 +57905,37 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="5" label="FullScreenActivity"/>
 </enum>
 
+<enum name="AndroidActivityStopReason" type="int">
+  <int value="0" label="Unknown"/>
+  <int value="1"
+      label="User hit the system back button, sending Chrome to the
+             background"/>

[Ilya Sherman, 2016/02/25 21:41:25]

Optional nit: These labels are pretty long.  It might be better to write this as

  <int value="1" label="User pressed back button">The user pressed the system
back button, sending Chrome to the background.</int>

That way, the label text displayed in the tabular view will be fairly brief, and
the more elaborate explanation will be available as hover text.

That said, there's always some tension between providing brief labels, and
providing sufficiently clear labels... and many (most?) users will not notice
the hover text.  So, I'll trust your judgement.  Just keep in mind that the
labels will be displayed as the left column of a tabular view.

[gone, 2016/02/25 22:18:14]

How's this?

+  <int value="2"
+      label="User hit the return UI button to close the tab and return to the
+             previous app"/>
+  <int value="3" label="Launched a child CustomTabActivity"/>
+  <int value="4" label="Child CustomTabActivity closed itself"/>
+  <int value="5"
+      label="Another of Chrome's Activities (e.g. Settings or Bookmarks) took
+             focus"/>
+</enum>
+
+<enum name="AndroidActivitySystemBackAction" type="int">
+  <int value="0" label="Nothing happened"/>
+  <int value="1"
+      label="The foreground tab contained a help article and was closed"/>
+  <int value="2"
+      label="Chrome was minimized and the foreground tab was kept open"/>
+  <int value="3"
+      label="Chrome was minimized and the foreground tab was closed"/>
+  <int value="4"
+      label="The foreground tab was closed, but Chrome stayed visible"/>
+  <int value="5" label="Error: No tab is currently being displayed"/>
+  <int value="6" label="User exited the tab switcher"/>
+  <int value="7" label="User exited fullscreen mode"/>
+  <int value="8" label="User navigated backward in the tab's history"/>
+</enum>
+
 <enum name="AndroidArmFpu" type="int">
   <int value="0" label="No NEON support"/>
   <int value="1" label="NEON support"/>