fix(ngMocks): $logProvider should not use internal APIs

angular.mocks.$LogProvider $logProvider.debugEnabled(false) is crashing
with undefined when run inside karma/jasmine test runner:

angular.module('foo', []).config(['$logProvider', function ($logProvider) {
  $logProvider.debugEnabled(false);
}]);

Closes #3612

src/ngMock/angular-mocks.js

@@ -316,7 +316,7 @@ angular.mock.$LogProvider = function() {
   }
 
   this.debugEnabled = function(flag) {
-	  if (isDefined(flag)) {
+	  if (angular.isDefined(flag)) {
 		  debug = flag;
 		  return this;
 	  } else {