Bug 1392688 Make variable DebugOnly to silence warning about it being unused r=jld

MozReview-Commit-ID: 7wSkP2Lcd0W

--HG--
extra : rebase_source : 1aa49424442f9bc278c257a3a65d93b22f04aea8

ipc/mscom/EnsureMTA.cpp

@@ -33,7 +33,7 @@ public:
   BackgroundMTAData()
   {
     nsCOMPtr<nsIRunnable> runnable = new EnterMTARunnable();
-    nsresult rv = NS_NewNamedThread("COM MTA",
+    mozilla::DebugOnly<nsresult> rv = NS_NewNamedThread("COM MTA",
                                     getter_AddRefs(mThread), runnable);
     NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "NS_NewNamedThread failed");
     MOZ_ASSERT(NS_SUCCEEDED(rv));