Add UMA metrics to log attachment broker errors.

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 1cc4c308156fc58f4afdcc3bb7badb4661e7555c..e9b5ce931d004fdeb47dea427cf46dc4a7a404fb 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -16165,6 +16165,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Network channel used for invalidations.</summary>
 </histogram>
 
+<histogram name="IPC.AttachmentBrokerPrivileged.BrokerAttachmentError"
+    enum="IPCAttachmentBrokerPrivilegedBrokerAttachmentError">
+  <owner>erikchen@chromium.org</owner>
+  <summary>
+    Errors that the privileged attachment broker encounters while trying to
+    broker attachments.
+  </summary>
+</histogram>
+
 <histogram name="Keyboard.KeystrokeDeltas" units="milliseconds">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>
@@ -63804,6 +63813,19 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="2" label="SubframeBlocked"/>
 </enum>
 
+<enum name="IPCAttachmentBrokerPrivilegedBrokerAttachmentError" type="int">
+  <int value="0" label="NO_DESTINATION">
+    The brokerable attachment did not have a destination process.
+  </int>
+  <int value="1" label="DESTINATION_NOT_FOUND">
+    The brokerable attachment had a destination, but the broker did not have a
+    channel of communication with that process.
+  </int>
+  <int value="2" label="DESTINATION_FOUND">
+    The brokerable attachment had a valid destination.
+  </int>

[Ilya Sherman, 2015/09/21 20:34:26]

Is this label the "success" case?  If so, I'd recommend moving it to be first in
the list.  That way, if you ever add additional error cases, it won't get stuck
in the middle.

If this isn't the success case, then I'd recommend adding a success case, so
that you understand the error rate, as well as the relative frequency of
different types of errors.

[erikchen, 2015/09/22 21:46:34]

It's the success case. I moved it to 0.

+</enum>
+
 <enum name="IPv6ConnectivityStatus" type="int">
   <int value="0" label="Incomplete IPv6 Configuration"/>
   <int value="1" label="Complete IPv6 Configuration"/>