sync: don't require a MessageLoopProxy in AttachmentServiceProxyForTest

sync/api/attachments/attachment_service_proxy_for_test.h

Index: sync/api/attachments/attachment_service_proxy_for_test.h
diff --git a/sync/api/attachments/attachment_service_proxy_for_test.h b/sync/api/attachments/attachment_service_proxy_for_test.h
index 916e788d74beba54597f4b6f6a7051505cf35bc5..2fb5c861e6a21a396b2740c2f0ece6cae4781e53 100644
--- a/sync/api/attachments/attachment_service_proxy_for_test.h
+++ b/sync/api/attachments/attachment_service_proxy_for_test.h
@@ -14,8 +14,9 @@ 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: