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..6a960e86b4f227230d478ca261c7f4436b88539d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -476,6 +476,13 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Apps.AppShimError" enum="AppShimError">
+  <owner>jackhou@chromium.org</owner>
+  <summary>
+    Counts which versions of app_mode_loader are sending --app-shim-error.

[tapted, 2014/07/01 05:53:59]

nit: Histograms folks usually ask for more details in this. Maybe something like
~"--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"

[jackhou1, 2014/07/01 07:37:51]

Done.

+  </summary>
+</histogram>
+
 <histogram name="Ash.ActiveTouchPoints">
   <owner>kuscher@google.com</owner>
   <owner>rbyers@chromium.org</owner>
@@ -34220,6 +34227,12 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="4" label="PROMO_SEEN"/>
 </enum>
 
+<enum name="AppShimError" type="int">
+  <int value="0" label="Shim was version 37 or lower."/>
+  <int value="1" label="Shim was version 38."/>
+  <int value="2" label="Shim was version 39 or higher."/>
+</enum>
+
 <enum name="AppsPageDragSource" type="int">
   <int value="0" label="Same apps pane"/>
   <int value="1" label="Different apps pane"/>