Implement InstanceIDAndroid using InstanceIDWithSubtype.java

components/gcm_driver/instance_id/instance_id.h

Index: components/gcm_driver/instance_id/instance_id.h
diff --git a/components/gcm_driver/instance_id/instance_id.h b/components/gcm_driver/instance_id/instance_id.h
index fb69a11ef7776187fd5b9695521b8d55a355c3ea..de9bc215aa71e216f7ffc307b65f938f41ebaff5 100644
--- a/components/gcm_driver/instance_id/instance_id.h
+++ b/components/gcm_driver/instance_id/instance_id.h
@@ -57,7 +57,7 @@ class InstanceID {
   // Creator.
   // |app_id|: identifies the application that uses the Instance ID.
   // |handler|: provides the GCM functionality needed to support Instance ID.
-  //            Must outlive this class.
+  //            Must outlive this class. On Android, this can be null instead.

[jianli, 2016/04/08 23:20:00]

Probably it is better to refactor this in order to avoid having different value
for a parameter. I am thinking:
  ... Create(const std::string& app_id, gcm::GCMDriver* gcm_driver)

InstanceIDAndroid could ignore gcm_driver parameter while InstanceIDImpl could
get InstanceIDHandler out of it.

[johnme, 2016/04/13 11:42:12]

Hmm, Peter suggested we pass in only the InstanceIDHandler in
https://codereview.chromium.org/1785613006, to insulate InstanceID from the
implementation detail that InstanceIDHandler happens to be obtained from &
implemented by GCMDriver (which is a different component). That still seems a
good reason.

This patch doesn't change this behaviour anyway (it was already null on Android,
I'm just documenting that fact).

   static scoped_ptr<InstanceID> Create(const std::string& app_id,
                                        gcm::InstanceIDHandler* handler);
 
@@ -106,17 +106,11 @@ class InstanceID {
   std::string app_id() const { return app_id_; }
 
  protected:
-  InstanceID(const std::string& app_id, gcm::InstanceIDHandler* handler);
+  InstanceID(const std::string& app_id);
 
   void NotifyTokenRefresh(bool update_id);
 
-  gcm::InstanceIDHandler* handler() const { return handler_; }
-
  private:
-  // Owned by GCMProfileServiceFactory, which is a dependency of
-  // InstanceIDProfileServiceFactory, which owns this.
-  gcm::InstanceIDHandler* handler_;
-
   std::string app_id_;
   TokenRefreshCallback token_refresh_callback_;