New tabCapture.captureOffscreenTab API, initially for Presentation API 1UA mode

chrome/renderer/resources/extensions/tab_capture_custom_bindings.js

 // Copyright (c) 2012 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.
 
 // Custom binding for the Tab Capture API.
 
 var binding = require('binding').Binding.create('tabCapture');
 
 binding.registerCustomHook(function(bindingsAPI, extensionId) {
   var apiFunctions = bindingsAPI.apiFunctions;
 
-  apiFunctions.setCustomCallback('capture',
-      function(name, request, callback, response) {
+  function proxyToGetUserMedia(name, request, callback, response) {
     if (!callback)
       return;
 
+    // TODO(miu): Propagate exceptions and always provide a useful error when
+    // callback() is invoked with a null argument.  http://crbug.com/463679
     if (response) {
       var options = {};
       if (response.audioConstraints)
         options.audio = response.audioConstraints;
       if (response.videoConstraints)
         options.video = response.videoConstraints;
 
       try {
         navigator.webkitGetUserMedia(options,
                                      function(stream) { callback(stream); },
-                                     function() { callback(null); });
+                                     function(exception) { callback(null); });
       } catch (e) {
         callback(null);
       }
     } else {
       callback(null);
     }
-  });
+  }
+
+  apiFunctions.setCustomCallback('capture', proxyToGetUserMedia);
+  apiFunctions.setCustomCallback('captureOffscreenTab', proxyToGetUserMedia);
 });
 
 exports.binding = binding.generate();