perf($compile): use strict comparison for controller === '@'

In the DDO, controller can be '@', which means the controller name
is taken from the directive attribute. This is undocumented and internally
only used by ngController. There seems to be no case where converting the
controller function to a string would actually be necessary.

Related #14268

src/ng/compile.js

@@ -2484,7 +2484,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
         };
 
         var controller = directive.controller;
-        if (controller == '@') {
+        if (controller === '@') {
           controller = attrs[directive.name];
         }