Add UMA metrics to log attachment broker errors.

ipc/attachment_broker_privileged.h

Index: ipc/attachment_broker_privileged.h
diff --git a/ipc/attachment_broker_privileged.h b/ipc/attachment_broker_privileged.h
index e8c675574c500a4766395e20bcff60e167c8e9d0..e1a4799e95d8dcbcd15298fa29a4b418c9029d2f 100644
--- a/ipc/attachment_broker_privileged.h
+++ b/ipc/attachment_broker_privileged.h
@@ -36,6 +36,20 @@ class IPC_EXPORT AttachmentBrokerPrivileged : public IPC::AttachmentBroker {
   // Returns nullptr if no sender is found.
   Sender* GetSenderWithProcessId(base::ProcessId id);
 
+  // Errors that can be reported by subclasses.
+  // These match tools/metrics/histograms.xml

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

nit: Please document that this enum should be treated as append-only

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

Done.

+  enum UMAError {
+    // The brokerable attachment did not have a destination process.
+    NO_DESTINATION = 0,
+    // The brokerable attachment had a destination, but the broker did not have
+    // a channel of communication with that process.
+    DESTINATION_NOT_FOUND,
+    MAX_VALUE

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

nit: "MAX_VALUE" is a little unclear out of context.  Perhaps "ERROR_MAX"?

[erikchen, 2015/09/19 01:21:55]

Done.

+  };
+
+  // Emits an UMA metric.
+  void LogError(UMAError error);
+
  private:
   std::vector<Endpoint*> endpoints_;
   DISALLOW_COPY_AND_ASSIGN(AttachmentBrokerPrivileged);