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 46950fa1c40c546257ea96f0fe19104718a4105c..abf2373b2eb2139b81b0f65b0195dee070a24ccd 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -476,6 +476,18 @@ 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 major milestone 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. For example, when Chrome updates from 32-bit to
+    64-bit, an older shim will find the new framework version but fail to dyload
+    it.
+  </summary>
+</histogram>
+
 <histogram name="Ash.ActiveTouchPoints">
   <owner>kuscher@google.com</owner>
   <owner>rbyers@chromium.org</owner>