Added better error and OAuth support in xhr.js.

remoting/webapp/js_proto/sinon_proto.js

Index: remoting/webapp/js_proto/sinon_proto.js
diff --git a/remoting/webapp/js_proto/sinon_proto.js b/remoting/webapp/js_proto/sinon_proto.js
index 9e223dc8eecc1afa9cf852c8dce89f32402e0f4d..142810674a060067948c7d7061ea10afb1c2cfa7 100644
--- a/remoting/webapp/js_proto/sinon_proto.js
+++ b/remoting/webapp/js_proto/sinon_proto.js
@@ -127,7 +127,15 @@ sinon.TestStub.prototype.onFirstCall = function() {};
 /** @returns {Object}  */
 sinon.createStubInstance = function (/** * */ constructor) {};
 
-/** @return {sinon.FakeXhr} */
+/** @interface */
+sinon.FakeXhrCtrl = function() {};
+
+/**
+ * @type {?function(!sinon.FakeXhr)}
+ */
+sinon.FakeXhrCtrl.prototype.onCreate;
+
+/** @return {sinon.FakeXhrCtrl} */
 sinon.useFakeXMLHttpRequest = function() {};
 
 /** @interface */
@@ -154,8 +162,3 @@ sinon.FakeXhr.prototype.requestHeaders;
  * @param {?string} content
  */
 sinon.FakeXhr.prototype.respond;
-
-/**
- * @type {?function(!sinon.FakeXhr)}
- */
-sinon.FakeXhr.prototype.onCreate;