Bug 1428242. P1 - assert we always take the MediaCache monitor off the main thread. r=bechen,gerald

MozReview-Commit-ID: 6fxhzTOFaWx

--HG--
extra : rebase_source : ad507e823ed90356e6b1b7452460fa8462240021
extra : intermediate-source : 1dc9fa1ba116ef65fddc076bf5508701d85d788b
extra : source : d5a6e82b6f344c59814c30b950577fd67ee0b7e3

dom/media/MediaCache.cpp

@@ -248,7 +248,11 @@ public:
   void Verify(AutoLock&) {}
 #endif
 
-  ReentrantMonitor& Monitor() { return mMonitor; }
+  ReentrantMonitor& Monitor()
+  {
+    MOZ_DIAGNOSTIC_ASSERT(!NS_IsMainThread());
+    return mMonitor;
+  }
 
   /**
    * An iterator that makes it easy to iterate through all streams that