Expose internal treegrid role, do some cleanup for table/grid/treegrid handling

chrome/test/data/extensions/api_test/automation/tests/tabs/attributes.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 var ActiveDescendantAttribute = [ 'activeDescendant' ];
 var LinkAttributes = [ 'url' ];
 var DocumentAttributes = [ 'docUrl',
                            'docTitle',
                            'docLoaded',
                            'docLoadingProgress' ];
@@ skipping 150 matching lines @@
     assertEq(1, progressIndicators[0].maxValueForRange);
 
     assertEq(0, scrollBars[0].valueForRange);
     assertEq(0, scrollBars[0].minValueForRange);
     assertEq(1, scrollBars[0].maxValueForRange);
 
     chrome.test.succeed();
   },
 
   function testTableAttributes() {
-    var table = rootNode.find({ role: 'table' });;
+    var table = rootNode.find({ role: 'grid' });;
     assertEq(3, table.tableRowCount);
     assertEq(3, table.tableColumnCount);
 
     var row1 = table.firstChild;
     var cell1 = row1.firstChild;
     assertEq(0, cell1.tableCellColumnIndex);
     assertEq(1, cell1.tableCellColumnSpan);
     assertEq(0, cell1.tableCellRowIndex);
     assertEq(1, cell1.tableCellRowSpan);
 
@@ skipping 94 matching lines @@
     // Uncheckable nodes have a checked attribute of undefined
     var checkTest4 = rootNode.find({ attributes: { name: 'check-test-4' } });
     assertTrue(Boolean(checkTest4));
     assertEq(checkTest4.checked, undefined);
 
     chrome.test.succeed();
   },
 ];
 
 setUpAndRunTests(allTests, 'attributes.html');