[Android WebAPKs] Add UMA metric for ShellAPK version

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 c5a6a1051d3ab145f47801f6850103b3fd4ca7ec..826febc83b22f71250a57c5f8b77e439e7a386aa 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -78496,6 +78496,26 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="WebApk.ShellApkVersion.BrowserApk">
+  <owner>hanxi@chromium.org</owner>
+  <owner>pkotwicz@chromium.org</owner>
+  <owner>yfriedman@chromium.org</owner>
+  <summary>
+    Records WebAPK &quot;Shell APK version&quot; when the WebAPK is launched.
+    Only recorded for WebAPKs installed by Chrome.

[Ilya Sherman, 2017/05/04 19:55:08]

Could you add a link to some documentation on how to understand Shell APK
versions?  (Alternately, would it make sense to associate an enum attribute with
this histogram?)

[pkotwicz, 2017/05/05 21:26:24]

I now mention chrome/android/webapk/shell_apk/shell_apk_version.gni in the
histogram. That file contains documentation about the version

+  </summary>
+</histogram>
+
+<histogram name="WebApk.ShellApkVersion.UnboundApk">

[Ilya Sherman, 2017/05/04 19:55:08]

Optional: You could use a histogram_suffixes element to avoid copy/pasting some
of this metadata.  The savings are pretty minor in this particular case, though.

+  <owner>hanxi@chromium.org</owner>
+  <owner>pkotwicz@chromium.org</owner>
+  <owner>yfriedman@chromium.org</owner>
+  <summary>
+    Records WebAPK &quot;Shell APK version&quot; when the WebAPK is launched.
+    Only recorded for WebAPKs which were not installed by Chrome.
+  </summary>
+</histogram>
+
 <histogram name="WebApk.Update.RequestQueued" enum="WebApkUpdateRequestQueued">
   <owner>hanxi@chromium.org</owner>
   <owner>pkotwicz@chromium.org</owner>