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

components/gcm_driver/gcm_driver.h

Index: components/gcm_driver/gcm_driver.h
diff --git a/components/gcm_driver/gcm_driver.h b/components/gcm_driver/gcm_driver.h
index f115d16562a00f5259063fee96ca738bf8819c74..074cea1a10568dc2b9ee2178f10e706f3602d21a 100644
--- a/components/gcm_driver/gcm_driver.h
+++ b/components/gcm_driver/gcm_driver.h
@@ -18,6 +18,7 @@
 #include "components/gcm_driver/crypto/gcm_encryption_provider.h"
 #include "components/gcm_driver/default_gcm_app_handler.h"
 #include "components/gcm_driver/gcm_client.h"
+#include "components/gcm_driver/gcm_message_status.h"
 
 namespace base {
 class FilePath;
@@ -199,7 +200,7 @@ class GCMDriver {
   // Updates the |account_mapping| information in persistent store.
   virtual void UpdateAccountMapping(const AccountMapping& account_mapping) = 0;
 
-  // Removes the account mapping information reated to |account_id| from
+  // Removes the account mapping information related to |account_id| from
   // persistent store.
   virtual void RemoveAccountMapping(const std::string& account_id) = 0;
 
@@ -290,9 +291,11 @@ class GCMDriver {
   // Dispatches the OnMessage event to the app handler associated with |app_id|.
   // If |message| has been encrypted, it will be decrypted asynchronously and
   // dispatched when the decryption operation was successful. Otherwise, the
-  // |message| will be dispatched immediately to the handler for |app_id|.
+  // |message| will be dispatched immediately to the handler for |app_id|. The
+  // |optional_receipt_callback| must be invoked on the UI thread.
   void DispatchMessage(const std::string& app_id,
-                       const IncomingMessage& message);
+                       const IncomingMessage& message,
+                       const MessageReceiptCallback& optional_receipt_callback);
 
  private:
   // Common code shared by Unregister and UnregisterWithSenderId.
@@ -303,9 +306,11 @@ class GCMDriver {
   // Dispatches the OnMessage event to the app handler associated with |app_id|
   // if |result| indicates that it is safe to do so, or will report a decryption
   // failure for the |app_id| otherwise.
-  void DispatchMessageInternal(const std::string& app_id,
-                               GCMEncryptionProvider::DecryptionResult result,
-                               const IncomingMessage& message);
+  void DispatchMessageInternal(
+      const std::string& app_id,
+      const MessageReceiptCallback& optional_receipt_callback,
+      GCMEncryptionProvider::DecryptionResult result,
+      const IncomingMessage& message);
 
   // Called after unregistration completes in order to trigger the pending
   // registration.