Fix the way the SDK interprets UIWebViewDelegate errors

Summary:
Currently if the facebook app encounters a UIWebViewDelegate error, it passes
it to the sdk in the error_code parameter. The SDK should know that if this
parameter is passed, an error occurred, which implies the user *didn't* cancel.
This fixes it.

Test Plan:
disconnected my laptop from the internet, went through the sso flow, verified
the sdk set userDidCancel to NO before calling [self
fbDialogNotLogin:userDidCancel].

DiffCamp Revision: 180310
Reviewed By: brent
CC: brent
Revert Plan:
OK

src/Facebook.m

@@ -251,22 +251,28 @@ - (BOOL)handleOpenURL:(NSURL *)url {
   // If the URL doesn't contain the access token, an error has occurred.
   if (!accessToken) {
     NSString *errorReason = [params valueForKey:@"error"];
-
+    
     // If the error response indicates that we should try again using Safari, open
     // the authorization dialog in Safari.
     if (errorReason && [errorReason isEqualToString:@"service_disabled_use_browser"]) {
       [self authorizeWithFBAppAuth:NO safariAuth:YES];
       return YES;
     }
-
+    
     // If the error response indicates that we should try the authorization flow
     // in an inline dialog, do that.
     if (errorReason && [errorReason isEqualToString:@"service_disabled"]) {
       [self authorizeWithFBAppAuth:NO safariAuth:NO];
       return YES;
     }
 
-    BOOL userDidCancel = !errorReason || [errorReason isEqualToString:@"access_denied"];
+    // The facebook app may return an error_code parameter in case it
+    // encounters a UIWebViewDelegate error. This should not be treated
+    // as a cancel.
+    NSString *errorCode = [params valueForKey:@"error_code"];
+
+    BOOL userDidCancel =
+      !errorCode && (!errorReason || [errorReason isEqualToString:@"access_denied"]);
     [self fbDialogNotLogin:userDidCancel];
     return YES;
   }