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..333baa45e63f720833956ecec3360e0d0a15bb9e 100644
--- a/components/gcm_driver/gcm_account_mapper.h
+++ b/components/gcm_driver/gcm_account_mapper.h
@@ -14,6 +14,7 @@
 #include "base/memory/weak_ptr.h"
 #include "components/gcm_driver/gcm_app_handler.h"
 #include "components/gcm_driver/gcm_client.h"
+#include "components/gcm_driver/gcm_message_status.h"
 #include "google_apis/gcm/engine/account_mapping.h"
 
 namespace base {
@@ -29,10 +30,11 @@ extern const char kGCMAccountMapperAppId[];
 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)>
-      DispatchMessageCallback;
+  using AccountMappings = std::vector<AccountMapping>;
+  using DispatchMessageCallback = base::Callback<void(
+      const std::string& app_id,
+      const IncomingMessage& message,
+      const MessageReceiptCallback& optional_receipt_callback)>;
 
   explicit GCMAccountMapper(GCMDriver* gcm_driver);
   ~GCMAccountMapper() override;
@@ -61,7 +63,7 @@ class GCMAccountMapper : public GCMAppHandler {
  private:
   friend class GCMAccountMapperTest;
 
-  typedef std::map<std::string, OutgoingMessage> OutgoingMessages;
+  using OutgoingMessages = std::map<std::string, OutgoingMessage>;
 
   // Checks whether account mapper is ready to process new account tokens.
   bool IsReady();