GCM: Make InstanceIDHandler implementations use protected inheritance

This avoids accidental layering violations, by enforcing use of
GCMDriver::GetInstanceIDHandlerInternal (which is commented as "These
methods must only be used by the InstanceID system") to get access to
InstanceIDHandlers.

BUG=none

Review-Url: https://codereview.chromium.org/2610933002
Cr-Commit-Position: refs/heads/master@{#441425}

components/gcm_driver/gcm_driver_desktop.h

@@ -42,7 +42,7 @@ class GCMDelayedTaskController;
 
 // GCMDriver implementation for desktop and Chrome OS, using GCMClient.
 class GCMDriverDesktop : public GCMDriver,
-                         public InstanceIDHandler {
+                         protected InstanceIDHandler {
  public:
   GCMDriverDesktop(
       std::unique_ptr<GCMClientFactory> gcm_client_factory,

components/gcm_driver/instance_id/fake_gcm_driver_for_instance_id.h

@@ -20,7 +20,7 @@ class SequencedTaskRunner;
 namespace instance_id {
 
 class FakeGCMDriverForInstanceID : public gcm::FakeGCMDriver,
-                                   public gcm::InstanceIDHandler {
+                                   protected gcm::InstanceIDHandler {
  public:
   FakeGCMDriverForInstanceID();
   explicit FakeGCMDriverForInstanceID(
@@ -30,6 +30,17 @@ class FakeGCMDriverForInstanceID : public gcm::FakeGCMDriver,
   // FakeGCMDriver overrides:
   gcm::InstanceIDHandler* GetInstanceIDHandlerInternal() override;
 
+  const std::string& last_gettoken_app_id() const {
+    return last_gettoken_app_id_;
+  }
+  const std::string& last_gettoken_authorized_entity() const {
+    return last_gettoken_authorized_entity_;
+  }
+  const std::string& last_deletetoken_app_id() const {
+    return last_deletetoken_app_id_;
+  }
+
+ protected:
   // InstanceIDHandler overrides:
   void GetToken(const std::string& app_id,
                 const std::string& authorized_entity,
@@ -48,16 +59,6 @@ class FakeGCMDriverForInstanceID : public gcm::FakeGCMDriver,
       const std::string& app_id,
       const GetInstanceIDDataCallback& callback) override;
 
-  const std::string& last_gettoken_app_id() const {
-    return last_gettoken_app_id_;
-  }
-  const std::string& last_gettoken_authorized_entity() const {
-    return last_gettoken_authorized_entity_;
-  }
-  const std::string& last_deletetoken_app_id() const {
-    return last_deletetoken_app_id_;
-  }
-
  private:
   std::map<std::string, std::pair<std::string, std::string>> instance_id_data_;
   std::map<std::string, std::string> tokens_;