Bug 1356516 - Close channel before destroying GMPServiceChild - r=billm

mServiceChild is a UniquePtr, so nulling it will destroy the GMPServiceChild,
which will destroy the associated message channel. So we need to close the
channel first before it gets destroyed. (Just as it was correctly done in
Observe() above.)

MozReview-Commit-ID: INuHN2Is7bC

--HG--
extra : rebase_source : 2a927bb06dd8fb4f1114dc0b64025cbdddc7c133

dom/media/gmp/GMPServiceChild.cpp

@@ -426,6 +426,7 @@ GeckoMediaPluginServiceChild::RemoveGMPContentParent(GMPContentParent* aGMPConte
   if (mServiceChild) {
     mServiceChild->RemoveGMPContentParent(aGMPContentParent);
     if (mShuttingDownOnGMPThread && !mServiceChild->HaveContentParents()) {
+      mServiceChild->Close();
       mServiceChild = nullptr;
     }
   }