Add support of web.app as an auth domain (#2959)

Author: Chuan Ren

Firebase/Auth/Source/AuthProvider/Phone/FIRPhoneAuthProvider.m

@@ -66,10 +66,6 @@ typedef void (^FIRVerifyClientCallback)(FIRAuthAppCredential *_Nullable appCrede
  */
 typedef void (^FIRFetchAuthDomainCallback)(NSString *_Nullable authDomain,
                                            NSError *_Nullable error);
-/** @var kAuthDomainSuffix
-    @brief The suffix of the auth domain pertiaining to a given Firebase project.
- */
-static NSString *const kAuthDomainSuffix = @"firebaseapp.com";
 
 /** @var kauthTypeVerifyApp
     @brief The auth type to be specified in the app verification request.

Firebase/Auth/Source/Utilities/FIRAuthWebUtils.m

@@ -23,13 +23,12 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
-/** @var kAuthDomainSuffix
-    @brief The suffix of the auth domain pertiaining to a given Firebase project.
- */
-static NSString *const kAuthDomainSuffix = @"firebaseapp.com";
-
 @implementation FIRAuthWebUtils
 
++ (NSArray<NSString *> *)supportedAuthDomains {
+  return @[@"firebaseapp.com", @"web.app"];
+}
+
 + (NSString *)randomStringWithLength:(NSUInteger)length {
   NSMutableString *randomString = [[NSMutableString alloc] init];
   for (int i=0; i < length; i++) {
@@ -99,11 +98,13 @@ + (void)fetchAuthDomainWithRequestConfiguration:(FIRAuthRequestConfiguration *)r
     }
     NSString *authDomain;
     for (NSString *domain in response.authorizedDomains) {
-      NSInteger index = domain.length - kAuthDomainSuffix.length;
-      if (index >= 2) {
-        if ([domain hasSuffix:kAuthDomainSuffix] && domain.length >= kAuthDomainSuffix.length + 2) {
-          authDomain = domain;
-          break;
+      for (NSString *suportedAuthDomain in [self supportedAuthDomains]) {
+        NSInteger index = domain.length - suportedAuthDomain.length;
+        if (index >= 2) {
+          if ([domain hasSuffix:suportedAuthDomain] && domain.length >= suportedAuthDomain.length + 2) {
+            authDomain = domain;
+            break;
+          }
         }
       }
     }