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

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

 description('Tests if the spellchecker behaves correctly when the spellcheck attribute '
     + 'is being changed by the script.');
 
 jsTestIsAsync = true;
+if (window.testRunner)
+    testRunner.setMockSpellCheckerEnabled(true);
 
 var parent = document.createElement("div");
 document.body.appendChild(parent);
 var sel = document.getSelection();
 
 function testSpellCheckingEnabled(target, enabled)
 {
     target.spellcheck = enabled;
 
     if (target.tagName == "SPAN") {
@@ skipping 65 matching lines @@
         setTimeout(testFor(currentElement, spellcheckAttributeVariances[iterator++]), 0);
     else {
         iterator = 0;
         currentElement = null;
         done();
     }
 }
 done();
 
 var successfullyParsed = true;