Bug 1497160 - Upgrade assertion type. r=baku.

Differential Revision: https://phabricator.services.mozilla.com/D7988

MozReview-Commit-ID: KPo6YHEcYYZ

dom/base/EventSource.cpp

@@ -149,7 +149,7 @@ public:
 
   void AssertIsOnTargetThread() const
   {
-    MOZ_ASSERT(IsTargetThread());
+    MOZ_DIAGNOSTIC_ASSERT(IsTargetThread());
   }
 
   bool IsTargetThread() const