sync: don't require a MessageLoopProxy in AttachmentServiceProxyForTest

sync/api/attachments/attachment_service_proxy_for_test.h

 // 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.
 
 #ifndef SYNC_API_ATTACHMENTS_ATTACHMENT_SERVICE_PROXY_FOR_TEST_H_
 #define SYNC_API_ATTACHMENTS_ATTACHMENT_SERVICE_PROXY_FOR_TEST_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "sync/api/attachments/attachment_service_proxy.h"
 #include "sync/base/sync_export.h"
 
 namespace syncer {
 
 // An self-contained AttachmentServiceProxy to reduce boilerplate code in tests.
 //
-// Constructs and owns an AttachmentService suitable for use in tests.  Assumes
-// the current thread has a MessageLoop.
+// Constructs and owns an AttachmentService suitable for use in tests.
+// NOTE: This class does not require the current thread to have a MessageLoop,
+// however all methods will effectively become no-op stubs in that case.
 class SYNC_EXPORT AttachmentServiceProxyForTest
     : public AttachmentServiceProxy {
  public:
   static AttachmentServiceProxy Create();
   virtual ~AttachmentServiceProxyForTest();
 
  private:
   // A Core that owns the wrapped AttachmentService.
   class OwningCore : public AttachmentServiceProxy::Core {
    public:
@@ skipping 12 matching lines @@
   };
 
   AttachmentServiceProxyForTest(
       const scoped_refptr<base::SequencedTaskRunner>& wrapped_task_runner,
       const scoped_refptr<Core>& core);
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_SERVICE_PROXY_FOR_TEST_H_