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 0a816b759399ab6a676a250812cd6efb8be6c475..65f3c2297f6c921305b8e081b0d9805a5108baba 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -15110,6 +15110,15 @@ 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>
@@ -60781,6 +60790,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="2" label="SubframeBlocked"/>
 </enum>
 
+<enum name="IPCAttachmentBrokerPrivilegedBrokerAttachmentError" type="int">

[Ilya Sherman, 2015/08/11 21:22:16]

I'd recommend adding either a "success" or an "attempt" bucket, so that you have
a baseline to compare to and be able to compute an error rate.

[erikchen, 2015/09/19 01:21:56]

I added a success bucket.

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