Add webview sample for captureVisibleRegion().

chrome/common/extensions/docs/examples/api/webview/capturevisibleregion/main.js

-// Copyright 2014 The Chromium Authors. All rights reserved.
+// Copyright (c) 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.
 
 chrome.app.runtime.onLaunched.addListener(function() {
-  chrome.app.window.create('hello.html', {
-    'innerBounds': {
-      'width': 400,
-      'height': 300
+  chrome.app.window.create('test.html', {
+    innerBounds: {
+      'width': 1280,
+      'height': 800
     }
   });
 });