Add UMA for --app-shim-error.

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 cfa152a82e82c983a69e7ed2bff3375856d50506..4832cfa1dced609f371dc5dfbf8ac4e79494eab5 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -476,6 +476,16 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Apps.AppShimErrorVersion">
+  <owner>jackhou@chromium.org</owner>
+  <summary>
+    Counts which versions of app_mode_loader are sending --app-shim-error.
+    --app-shim-error is sent as a command line argument to Chrome when
+    app_mode_loader was unable to dyload the Chrome Framework and call
+    ChromeAppModeMain.

[Alexei Svitkine (slow), 2014/07/04 12:49:24]

Please clarify this comment to mention what "version" means. It looks like it's
the major milestone version - if so, please say so.

Also, please update the comment to explain or give an example of when this is
expected to be different from Chrome's version number that's logging this error.
(If it can't be different, then why bucketize by version?)

[jackhou1, 2014/07/07 05:38:36]

Done.

+  </summary>
+</histogram>
+
 <histogram name="Ash.ActiveTouchPoints">
   <owner>kuscher@google.com</owner>
   <owner>rbyers@chromium.org</owner>