Issue 475: Fix a bug when perssing on back button

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

Index: Src/GoogleApis.Auth.WP/OAuth2/WebAuthenticationBrokerUserControl.xaml.cs
===================================================================
--- a/Src/GoogleApis.Auth.WP/OAuth2/WebAuthenticationBrokerUserControl.xaml.cs
 b/Src/GoogleApis.Auth.WP/OAuth2/WebAuthenticationBrokerUserControl.xaml.cs
@@ -96,6 96,10 @@
 public Task<AuthorizationCodeResponseUrl> Launch(Uri uri)
 {
 tcsAuthorizationCodeResponse = new TaskCompletionSource<AuthorizationCodeResponseUrl>();
 tcsAuthorizationCodeResponse.Task.ContinueWith(t =>
 {
 RemoveBackKeyPressCallback();
 }, TaskScheduler.FromCurrentSynchronizationContext());
 StartLoading();
 browser.Navigate(uri);
 return tcsAuthorizationCodeResponse.Task;
@@ -104,13 108,16 @@
 /// <summary>A callback handler for when the user presses the back key.</summary>
 void RootPage_BackKeyPress(object sender, System.ComponentModel.CancelEventArgs 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 PhoneApplicationPage;
 rootPage.BackKeyPress -= RootPage_BackKeyPress;
-
- e.Cancel = true;
- tcsAuthorizationCodeResponse.SetCanceled();
 }
 }
 }