Issue 475: Fix a bug when perssing on back button

Src/GoogleApis.Auth.WP/OAuth2/WebAuthenticationBrokerUserControl.xaml.cs

 /*
 Copyright 2013 Google Inc
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
 You may obtain a copy of the License at
 
 http://www.apache.org/licenses/LICENSE-2.0
 
 Unless required by applicable law or agreed to in writing, software
@@ skipping 78 matching lines @@
 {
 StopLoading();
 }
 
 /// <summary>The window launcher that starts browse the browser controll
er to the given URI.</summary>
 /// <param name="uri">The authorization code request URI</param>
 /// <returns>The authorization code response</returns>
 public Task<AuthorizationCodeResponseUrl> Launch(Uri uri)
 {
 tcsAuthorizationCodeResponse = new TaskCompletionSource<Authorizatio
nCodeResponseUrl>();
+tcsAuthorizationCodeResponse.Task.ContinueWith(t =>
+{
+RemoveBackKeyPressCallback();
+}, TaskScheduler.FromCurrentSynchronizationContext());
 StartLoading();
 browser.Navigate(uri);
 return tcsAuthorizationCodeResponse.Task;
 }
 
 /// <summary>A callback handler for when the user presses the back key.<
/summary>
 void RootPage_BackKeyPress(object sender, System.ComponentModel.CancelEv
entArgs e)
 {
- // Remove this callback.
+ e.Cancel = true;
+ tcsAuthorizationCodeResponse.SetCanceled();
+ }
+
+ /// <summary>Removes <see cref="RootPage_BackKeyPress" as the root page 
 callback./></summary>
+ void RemoveBackKeyPressCallback()
+ {
 PhoneApplicationFrame rootFrame = Application.Current.RootVisual as
PhoneApplicationFrame;
 PhoneApplicationPage rootPage = rootFrame.Content as PhoneApplicatio
nPage;
 rootPage.BackKeyPress -= RootPage_BackKeyPress;
-
-e.Cancel = true;
-tcsAuthorizationCodeResponse.SetCanceled();
 }
 }
 }