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..d3cf052087e26a450976b73851bbb49311188844 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="DESTINATION_FOUND">
+    The brokerable attachment had a valid destination. This is the success case.
+  </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="NO_DESTINATION">
+    The brokerable attachment did not have a destination process.
+  </int>
+</enum>
+
 <enum name="IPv6ConnectivityStatus" type="int">
   <int value="0" label="Incomplete IPv6 Configuration"/>
   <int value="1" label="Complete IPv6 Configuration"/>