Bug 1154802 - Tighten up some existing assertions. r=jww

This is a followup from bug 1154805 comment 13.

dom/media/AbstractThread.cpp

@@ -45,7 +45,7 @@ public:
   virtual bool IsCurrentThreadIn() override
   {
     bool in = NS_GetCurrentThread() == mTarget;
-    MOZ_ASSERT_IF(in, GetCurrent() == this);
+    MOZ_ASSERT(in == (GetCurrent() == this));
     return in;
   }
 

dom/media/MediaTaskQueue.cpp

@@ -220,7 +220,7 @@ bool
 MediaTaskQueue::IsCurrentThreadIn()
 {
   bool in = NS_GetCurrentThread() == mRunningThread;
-  MOZ_ASSERT_IF(in, GetCurrent() == this);
+  MOZ_ASSERT(in == (GetCurrent() == this));
   return in;
 }