Add a switch to TestRunner to enable/disable mock spell checker

third_party/WebKit/LayoutTests/editing/spelling/script-tests/spelling-attribute-at-child.js

 description('Tests if the spellchecker behaves correctly when child has own '
     + 'spellcheck attribute.');
 
 jsTestIsAsync = true;
+if (window.testRunner)
+    testRunner.setMockSpellCheckerEnabled(true);
 
 var root = document.createElement("div");
 document.body.appendChild(root);
 
 function verifyChildSpellingMarker(element)
 {
     var testElement = document.createElement("div");
     testElement.innerHTML = element.markup;
     root.appendChild(testElement);
 
@@ skipping 30 matching lines @@
 {
     var nextTestCase = testCases.shift();
     if (nextTestCase)
         return setTimeout(verifyChildSpellingMarker(nextTestCase), 0);
 
     finishJSTest();
 }
 done();
 
 var successfullyParsed = true;