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

components/gcm_driver/fake_gcm_driver.cc

Index: components/gcm_driver/fake_gcm_driver.cc
diff --git a/components/gcm_driver/fake_gcm_driver.cc b/components/gcm_driver/fake_gcm_driver.cc
index 6f4e5d863dade39fc64c50cd4d44bdfee4135d1d..fa7236315f8e2545b232bc3464a4a3de664ddb20 100644
--- a/components/gcm_driver/fake_gcm_driver.cc
+++ b/components/gcm_driver/fake_gcm_driver.cc
@@ -112,4 +112,8 @@ void FakeGCMDriver::AddHeartbeatInterval(const std::string& scope,
 void FakeGCMDriver::RemoveHeartbeatInterval(const std::string& scope) {
 }
 
+void FakeGCMDriver::DoSendMessageReceipt(const MessageReceiptCallback& callback,
+                                         GCMMessageStatus status) {
+  callback.Run(status);
+}
 }  // namespace gcm