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..4badf0f51b9192a00096aeda23af55a5c9ec1ab4 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 +174,14 @@ 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 can be invoked from any thread, but will
+    // run on the UI thread.

[Peter Beverloo, 2017/02/08 17:09:09]

[The callback can...the UI thread]

That seems wrong. The GCM Client lives on the IO thread, and since it uses
Send() it *must* be invoked on the IO thread.

[harkness, 2017/02/09 16:27:29]

Yup, I updated the code without updating the comment. I'll fix that.

+    virtual void OnMessageReceived(
+        const std::string& app_id,
+        const IncomingMessage& message,
+        const MessageReceiptCallback& optional_receipt_callback) = 0;
 
     // Called when some messages have been deleted from the server.
     // |app_id|: application ID.