Raise exception when log into Facebook using OAuth. (#3138)

Raise exception when log into Facebook using OAuth. Facebook requires use of its official SDK for mobile and doesn't allow web based flows.

Author: Yue-Wang-Google

Firebase/Auth/Source/AuthProvider/OAuth/FIROAuthProvider.m

@@ -23,9 +23,11 @@
 #import "FIRAuthBackend.h"
 #import "FIRAuth_Internal.h"
 #import "FIRAuthErrorUtils.h"
+#import "FIRAuthExceptionUtils.h"
 #import "FIRAuthGlobalWorkQueue.h"
 #import "FIRAuthRequestConfiguration.h"
 #import "FIRAuthWebUtils.h"
+#import "FIRFacebookAuthProvider.h"
 #import "FIROAuthCredential_Internal.h"
 #import "FIROAuthCredential.h"
 
@@ -165,6 +167,12 @@ - (void)getCredentialWithUIDelegate:(nullable id<FIRAuthUIDelegate>)UIDelegate
     @return An Instance of @c FIROAuthProvider.
   */
 - (nullable instancetype)initWithProviderID:(NSString *)providerID auth:(FIRAuth *)auth {
+  if ([providerID isEqual:FIRFacebookAuthProviderID]) {
+    [FIRAuthExceptionUtils raiseInvalidParameterExceptionWithReason:
+        @"Sign in with Facebook is not supported via this method; the Facebook TOS "
+         "dictate that you must use the Facebook iOS SDK for Facebook login."];
+    return nil;
+  }
   self = [super init];
   if (self) {
     _auth = auth;