Add UMA metrics to log attachment broker errors.

ipc/attachment_broker_privileged.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef IPC_ATTACHMENT_BROKER_PRIVILEGED_H_
 #define IPC_ATTACHMENT_BROKER_PRIVILEGED_H_
 
 #include <vector>
 
 #include "ipc/attachment_broker.h"
@@ skipping 18 matching lines @@
   // process, these channels must be registered and deregistered with the
   // Attachment Broker as they are created and destroyed.
   void RegisterCommunicationChannel(Endpoint* endpoint);
   void DeregisterCommunicationChannel(Endpoint* endpoint);
 
  protected:
   // Returns the sender whose peer's process id is |id|.
   // 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);
 };
 
 }  // namespace IPC
 
 #endif  // IPC_ATTACHMENT_BROKER_PRIVILEGED_H_