Revert to system Mail app if current default Mail app is no longer there

ios/chrome/browser/web/mailto_url_rewriter.mm

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/web/mailto_url_rewriter.h"
 
 #import "base/logging.h"
 #import "ios/chrome/browser/web/mailto_handler.h"
 #import "ios/chrome/browser/web/mailto_handler_gmail.h"
 #import "ios/chrome/browser/web/mailto_handler_system_mail.h"
@@ skipping 72 matching lines @@
   return [[_handlers allValues]
       sortedArrayUsingComparator:^NSComparisonResult(
           MailtoHandler* _Nonnull obj1, MailtoHandler* _Nonnull obj2) {
         return [[obj1 appName] compare:[obj2 appName]];
       }];
 }
 
 - (NSString*)defaultHandlerID {
   NSString* value = [[NSUserDefaults standardUserDefaults]
       stringForKey:kMailtoDefaultHandlerKey];
-  return value ? value : [[self class] systemMailApp];
+  if ([_handlers[value] isAvailable])
+    return value;
+  return [[self class] systemMailApp];
 }
 
 - (void)setDefaultHandlerID:(NSString*)appStoreID {
   DCHECK([appStoreID length]);
   NSUserDefaults* defaults = [NSUserDefaults standardUserDefaults];
   if ([appStoreID
           isEqualToString:[defaults objectForKey:kMailtoDefaultHandlerKey]])
     return;
   [defaults setObject:appStoreID forKey:kMailtoDefaultHandlerKey];
   [_observer rewriterDidChange:self];
@@ skipping 107 matching lines @@
   if ([defaults objectForKey:kMailtoDefaultHandlerKey])
     return;
 
   NSString* const kGmailAppStoreID = @"422689480";
   MailtoHandler* gmailHandler = _handlers[kGmailAppStoreID];
   if ([gmailHandler isAvailable])
     [defaults setObject:kGmailAppStoreID forKey:kMailtoDefaultHandlerKey];
 }
 
 @end