Bug 1299749 - removed unused variable from SessionMessageTask. r=baku

MozReview-Commit-ID: 6QfkkCfQ60

--HG--
extra : rebase_source : 8dd222b4218f4b461e547713c5e0f45998a3569c

dom/media/gmp/GMPCDMCallbackProxy.cpp

@@ -157,7 +157,6 @@ public:
   }
 
   RefPtr<CDMProxy> mProxy;
-  dom::PromiseId mPid;
   nsString mSid;
   dom::MediaKeyMessageType mMsgType;
   nsTArray<uint8_t> mMsg;