Add UMA metrics to track the source of homescreen icons on launch.

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 e4ace477fb2e30ed73b50560502e992543198fb2..2f8b6515369971b27be0f4bcca58111e4fc2add6 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -14691,6 +14691,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Launch.HomeScreenSource" enum="LaunchFromHomeScreenSource">
+  <owner>dominickn@chromium.org</owner>
+  <summary>
+    Records the source of an Android Home screen shortcut used to launch Chrome.
+  </summary>
+</histogram>
+
 <histogram name="Launch.InvalidIntent" enum="BooleanHit">
   <owner>dfalcantara@chromium.org</owner>
   <summary>
@@ -59359,6 +59366,12 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Launched as a tab"/>
 </enum>
 
+<enum name="LaunchFromHomeScreenSource" type="int">
+  <int value="0" label="Home screen shortcut source unknown"/>

[Ilya Sherman, 2015/07/03 18:11:45]

nit: These labels are going to be displayed as the left column of a tabular
layout, so shorter labels are better.  For example, I'd shorten this to just
"Unknown".

[dominickn, 2015/07/05 23:33:18]

Done.

+  <int value="1" label="Add to Home screen menu item"/>
+  <int value="2" label="App banner"/>
+</enum>
+
 <enum name="LevelDBCorruptionTypes" type="int">
   <int value="0" label="other"/>
   <int value="1" label="missing files"/>