Bug 1540136 - P4: Remove ChromiumCDMParent mMainThread as it is unused. r=cpearce

Differential Revision: https://phabricator.services.mozilla.com/D26207

--HG--
extra : moz-landing-system : lando

dom/media/gmp/ChromiumCDMParent.cpp

@@ -62,7 +62,6 @@ RefPtr<ChromiumCDMParent::InitPromise> ChromiumCDMParent::Init(
         __func__);
   }
   mCDMCallback = aCDMCallback;
-  mMainThread = aMainThread;
 
   RefPtr<ChromiumCDMParent::InitPromise> promise =
       mInitPromise.Ensure(__func__);

dom/media/gmp/ChromiumCDMParent.h

@@ -180,9 +180,6 @@ class ChromiumCDMParent final : public PChromiumCDMParent,
   // life time of this object, but never more than one active at once.
   uint32_t mMaxRefFrames = 0;
   ReorderQueue mReorderQueue;
-
-  // The main thread associated with the root document. Must be set in Init().
-  nsCOMPtr<nsIEventTarget> mMainThread;
 };
 
 }  // namespace gmp