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

chrome/renderer/resources/extensions/tab_capture_custom_bindings.js

Index: chrome/renderer/resources/extensions/tab_capture_custom_bindings.js
diff --git a/chrome/renderer/resources/extensions/tab_capture_custom_bindings.js b/chrome/renderer/resources/extensions/tab_capture_custom_bindings.js
index 93a60549e3743d366c47fabe14f7d8c63bff316d..b0fdedb8c4dcb89a3dc4ff6889b61fd77266391e 100644
--- a/chrome/renderer/resources/extensions/tab_capture_custom_bindings.js
+++ b/chrome/renderer/resources/extensions/tab_capture_custom_bindings.js
@@ -9,11 +9,12 @@ 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)
@@ -22,16 +23,33 @@ binding.registerCustomHook(function(bindingsAPI, extensionId) {
         options.video = response.videoConstraints;
 
       try {
-        navigator.webkitGetUserMedia(options,
-                                     function(stream) { callback(stream); },
-                                     function() { callback(null); });
+        navigator.webkitGetUserMedia(
+            options,
+            function(stream) {
+              if (stream && ('offscreenTabId' in response)) {
+                Object.defineProperty(stream, 'offscreenTabId', {
+                  configureable: false,
+                  enumerable: false,
+                  value: response['offscreenTabId'],
+                  writable: false
+                });
+              }
+              callback(stream);
+            },
+            function(exception) {
+              callback(null);
+            }
+        );
       } catch (e) {
         callback(null);
       }
     } else {
       callback(null);
     }
-  });
+  }
+
+  apiFunctions.setCustomCallback('capture', proxyToGetUserMedia);
+  apiFunctions.setCustomCallback('captureOffscreenTab', proxyToGetUserMedia);
 });
 
 exports.binding = binding.generate();