Add UMA metrics to log attachment broker errors.

ipc/attachment_broker_privileged.cc

Index: ipc/attachment_broker_privileged.cc
diff --git a/ipc/attachment_broker_privileged.cc b/ipc/attachment_broker_privileged.cc
index da62ac0c06e87fd5f1d7d050c021cfe039f6408f..e539f62159994d7cbab97b1936ff3c293fa0d27b 100644
--- a/ipc/attachment_broker_privileged.cc
+++ b/ipc/attachment_broker_privileged.cc
@@ -6,6 +6,7 @@
 
 #include <algorithm>
 
+#include "base/metrics/histogram.h"
 #include "ipc/ipc_endpoint.h"
 
 namespace IPC {
@@ -37,4 +38,9 @@ Sender* AttachmentBrokerPrivileged::GetSenderWithProcessId(base::ProcessId id) {
   return *it;
 }
 
+void AttachmentBrokerPrivileged::LogError(UMAError error) {
+  UMA_HISTOGRAM_ENUMERATION(
+      "IPC.AttachmentBrokerPrivileged.BrokerAttachmentError", error, MAX_VALUE);
+}
+
 }  // namespace IPC