Add support for GCM subtypes to desktop Instance ID implementation

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 add4deabf928426be6550db6b8c094db6c2195f6..88c855f54d43027625c3d62716e27015440f0c25 100644
--- a/components/gcm_driver/gcm_client.h
+++ b/components/gcm_driver/gcm_client.h
@@ -98,6 +98,7 @@ class GCMClient {
     ChromePlatform platform;
     ChromeChannel channel;
     std::string version;
+    std::string category_for_subtypes;
   };
 
   // Detailed information of the Send Error event.
@@ -318,6 +319,10 @@ class GCMClient {
                                  std::string* instance_id,
                                  std::string* extra_data) = 0;
 
+  // Returns whether this is the special category used for Instance IDs sending
+  // their |app_id| to GCM as a "subtype" (see security note at implementation).
+  virtual bool CategoryHasSubtype(const std::string& category) = 0;

[Peter Beverloo, 2016/07/22 12:17:03]

What's the benefit of keeping the ambiguously named "CategoryHasSubtype"
mentioning a "special category" if introducing the concept of a "product
subtype" would catch them all?

[johnme, 2016/07/26 17:11:55]

Fair. I've inlined this into GCMClientImpl::HandleIncomingMessage.

+
   // Gets and sets custom heartbeat interval for the MCS connection.
   // |scope| is used to identify the component that requests a custom interval
   // to be set, and allows that component to later revoke the setting. It should