Add metrics for tracking how Chrome handled shortcut launches

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 5c2ddf9bd9a784717d76ea4a8b649f86400f9d54..67933ce54ce9d0a05543dfd14150a26b87d56e87 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -11198,6 +11198,13 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="HomeScreen.Launch" enum="HomeScreenLaunch">

[Ilya Sherman, 2015/03/06 21:08:11]

It looks like you're creating a new "HomeScreen" group.  It's generally best to
try to keep the top-level namespace sparsely populated, and to group new
histograms into existing groups, where that makes sense.  Is there an existing
group that would make sense to use for this histogram?  (You might have already
thought about this, and decided that the answer is "no".  It just seems like
"HomeScreen" is hyper-specific, and not likely to garner any additional related
histograms.  I'm wondering if there's not a more general concept that this
metric falls under, which we'd expect to want to also measure.)

[gone, 2015/03/06 22:14:09]

Trying to group it under the top-level "Launch", which only has a "Launch.Modes"
in it.

+  <owner>dfalcantara@chromium.org</owner>
+  <summary>
+    Records how Chrome handled a launch from an Android Home screen shortcut.
+  </summary>
+</histogram>
+
 <histogram name="Hotword.AudioLoggingEnabled" enum="BooleanEnabled">
   <owner>rlp@chromium.org</owner>
   <summary>
@@ -50778,6 +50785,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </int>
 </enum>
 
+<enum name="HomeScreenLaunch" type="int">
+  <int value="0" label="Launched as standalone web app"/>
+  <int value="1" label="Launched as a tab"/>
+</enum>
+
 <enum name="HotwordAvailability" type="int">
   <int value="0" label="Unavailable -- reason may be unknown"/>
   <int value="1" label="Available"/>