Improve comment in OAuth2TokenServiceRequest::Core.

google_apis/gaia/oauth2_token_service_request.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "google_apis/gaia/oauth2_token_service_request.h"
 
 #include "base/bind.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/single_thread_task_runner.h"
@@ skipping 59 matching lines @@
   OAuth2TokenServiceRequest* owner();
 
  private:
   friend class base::RefCountedThreadSafe<OAuth2TokenServiceRequest::Core>;
 
   void DoNothing();
 
   scoped_refptr<base::SingleThreadTaskRunner> token_service_task_runner_;
   OAuth2TokenServiceRequest* owner_;
 
-  // It is important that provider_ is destroyed on the owner thread, not the
-  // token_service_task_runner_ thread.
+  // Clear on owner thread.  OAuth2TokenServiceRequest promises to clear its
+  // last reference to TokenServiceProvider on the owner thread so the caller
+  // can ensure it is destroyed on the owner thread if desired.
   scoped_refptr<TokenServiceProvider> provider_;
 
   DISALLOW_COPY_AND_ASSIGN(Core);
 };
 
 OAuth2TokenServiceRequest::Core::Core(
     OAuth2TokenServiceRequest* owner,
     const scoped_refptr<TokenServiceProvider>& provider)
     : owner_(owner), provider_(provider) {
   DCHECK(owner_);
@@ skipping 269 matching lines @@
     const std::string& account_id)
     : account_id_(account_id) {
   DCHECK(!account_id_.empty());
 }
 
 void OAuth2TokenServiceRequest::StartWithCore(const scoped_refptr<Core>& core) {
   DCHECK(core);
   core_ = core;
   core_->Start();
 }