Provide a mechanism for the GCM driver to send message receipts to GCM.

components/gcm_driver/gcm_account_mapper.h

Index: components/gcm_driver/gcm_account_mapper.h
diff --git a/components/gcm_driver/gcm_account_mapper.h b/components/gcm_driver/gcm_account_mapper.h
index 39cd4dd792e3969450a73cb3d06097be6413f16f..9587f59155d754a27e42673f4e6867fa707cfa51 100644
--- a/components/gcm_driver/gcm_account_mapper.h
+++ b/components/gcm_driver/gcm_account_mapper.h
@@ -30,8 +30,10 @@ class GCMAccountMapper : public GCMAppHandler {
  public:
   // List of account mappings.
   typedef std::vector<AccountMapping> AccountMappings;
-  typedef base::Callback<void(const std::string& app_id,
-                              const IncomingMessage& message)>
+  typedef base::Callback<void(
+      const std::string& app_id,
+      const IncomingMessage& message,
+      const GCMClient::MessageReceiptCallback& callback)>
       DispatchMessageCallback;
 
   explicit GCMAccountMapper(GCMDriver* gcm_driver);