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

components/gcm_driver/gcm_client.h

Index: components/gcm_driver/gcm_client.h
diff --git a/components/gcm_driver/gcm_client.h b/components/gcm_driver/gcm_client.h
index c4dd12adc914375e3c8cf118e114589f292a7df5..c158f8a82f4a9db50fae3bca7322a9f577423376 100644
--- a/components/gcm_driver/gcm_client.h
+++ b/components/gcm_driver/gcm_client.h
@@ -16,6 +16,7 @@
 #include "components/gcm_driver/common/gcm_messages.h"
 #include "components/gcm_driver/crypto/gcm_encryption_provider.h"
 #include "components/gcm_driver/gcm_activity.h"
+#include "components/gcm_driver/gcm_message_status.h"
 #include "components/gcm_driver/registration_info.h"
 
 template <class T> class scoped_refptr;
@@ -173,8 +174,13 @@ class GCMClient {
     // Called when a message has been received.
     // |app_id|: application ID.
     // |message|: message received.
-    virtual void OnMessageReceived(const std::string& app_id,
-                                   const IncomingMessage& message) = 0;
+    // |optional_receipt_callback|: Optional callback to invoke if desired when
+    // processing the message is complete if the receiver wants to return a
+    // delivery receipt. The callback must be invoked on the IO thread.
+    virtual void OnMessageReceived(
+        const std::string& app_id,
+        const IncomingMessage& message,
+        const MessageReceiptCallback& optional_receipt_callback) = 0;

[Peter Beverloo, 2017/02/13 13:47:06]

nit: standardize on (optional_)?receipt_callback naming. Distinguishing between
threads sgtm, but let's use the same otherwise.

[harkness, 2017/02/14 19:22:28]

I decided that I really like "optional" for standardization, even when there are
threading questions. I didn't want to switch back and forth from optional_ to
io_ or something like that.

 
     // Called when some messages have been deleted from the server.
     // |app_id|: application ID.