Add support for retrieving image thumbnails as part of the accessibility tree.

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

+// Copyright 2016 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 allTests = [
+  function testGetImageData() {
+    var image = rootNode.find({ role: RoleType.image });
+    image.addEventListener(EventType.imageFrameUpdated, function() {
+      assertEq(image.imageDataUrl.substr(0, 22), 'data:image/png;base64,');
+      var imgElement = document.createElement('img');
+      imgElement.src = image.imageDataUrl;
+      var canvas = document.createElement('canvas');
+      canvas.setAttribute('width', 2);
+      canvas.setAttribute('height', 3);
+      var context = canvas.getContext('2d');
+      context.drawImage(imgElement, 0, 0);
+      var imageData = context.getImageData(0, 0, 2, 3);
+      // Check image data in RGBA format.
+      // Top row: red
+      assertEq(imageData.data[0], 0xFF);
+      assertEq(imageData.data[1], 0x00);
+      assertEq(imageData.data[2], 0x00);
+      assertEq(imageData.data[3], 0xFF);
+      assertEq(imageData.data[4], 0xFF);
+      assertEq(imageData.data[5], 0x00);
+      assertEq(imageData.data[6], 0x00);
+      assertEq(imageData.data[7], 0xFF);
+      // Middle row: green
+      assertEq(imageData.data[8], 0x00);
+      assertEq(imageData.data[9], 0xFF);
+      assertEq(imageData.data[10], 0x00);
+      assertEq(imageData.data[11], 0xFF);
+      assertEq(imageData.data[12], 0x00);
+      assertEq(imageData.data[13], 0xFF);
+      assertEq(imageData.data[14], 0x00);
+      assertEq(imageData.data[15], 0xFF);
+      // Last row: blue
+      assertEq(imageData.data[16], 0x00);
+      assertEq(imageData.data[17], 0x00);
+      assertEq(imageData.data[18], 0xFF);
+      assertEq(imageData.data[19], 0xFF);
+      assertEq(imageData.data[20], 0x00);
+      assertEq(imageData.data[21], 0x00);
+      assertEq(imageData.data[22], 0xFF);
+      assertEq(imageData.data[23], 0xFF);
+      chrome.test.succeed();
+    }, true);
+    image.getImageData(0, 0);
+  }
+];
+
+setUpAndRunTests(allTests, 'image_data.html');