GCM Engine: Split up reg/unreg UNKNOWN_ERROR to improve metrics

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 df61dd23653b4a56b7aead841dbd7f485b517af0..6126a31e55fbcd6f0ee9f3e5d795e1ec5524099b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -18526,7 +18526,12 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="GCM.RegistrationRequestStatus"
     enum="GCMRegistrationRequestStatus">
   <owner>juyik@chromium.org</owner>
-  <summary>Status code of the outcome of a GCM registration request.</summary>
+  <summary>
+    Status code of the outcome of a GCM registration request. The Unknown error
+    case was split up in M56 (merged mid-beta to M55) to separate out the
+    Response parsing failed, Internal server error, Quota exceeded, and Device
+    has too many registrations cases.
+  </summary>
 </histogram>
 
 <histogram name="GCM.RegistrationRetryCount">
@@ -86761,6 +86766,10 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="7" label="HTTP not OK"/>
   <int value="8" label="No response body"/>
   <int value="9" label="Reached maximum number of retries"/>
+  <int value="10" label="Response parsing failed"/>
+  <int value="11" label="Internal server error"/>
+  <int value="12" label="Quota exceeded"/>
+  <int value="13" label="Device has too many registrations"/>
 </enum>
 
 <enum name="GCMResetStoreError" type="int">
@@ -86790,6 +86799,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="8" label="HTTP reponse code not OK"/>
   <int value="9" label="Unknown error"/>
   <int value="10" label="Reached maximum number of retries"/>
+  <int value="11" label="Device registration error"/>
 </enum>
 
 <enum name="GCMUpstreamMessageStatus" type="int">