Bug 1299727 - Rename NS_WARN_IF_FALSE as NS_WARNING_ASSERTION. r=erahm.

The new name makes the sense of the condition much clearer. E.g. compare:

  NS_WARN_IF_FALSE(!rv.Failed());

with:

  NS_WARNING_ASSERTION(!rv.Failed());

The new name also makes it clearer that it only has effect in debug builds,
because that's standard for assertions.

dom/html/HTMLMenuElement.cpp

@@ -113,7 +113,7 @@ HTMLMenuElement::CreateBuilder()
   }
 
   nsCOMPtr<nsIMenuBuilder> builder = do_CreateInstance(HTMLMENUBUILDER_CONTRACTID);
-  NS_WARN_IF_FALSE(builder, "No builder available");
+  NS_WARNING_ASSERTION(builder, "No builder available");
   return builder.forget();
 }