Correct iOS build for latest Xcode beta.

ios/chrome/browser/ui/commands/generic_chrome_command.mm

 // Copyright 2012 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/ui/commands/generic_chrome_command.h"
 
 #import <UIKit/UIKit.h>
 
 #include "base/logging.h"
 #import "ios/chrome/browser/ui/commands/UIKit+ChromeExecuteCommand.h"
 
 @implementation GenericChromeCommand
 
 @synthesize tag = _tag;
 
+- (instancetype)init {
+  NOTREACHED();
+  return nil;
+}
+
 - (instancetype)initWithTag:(NSInteger)tag {
   self = [super init];
   if (self) {
     _tag = tag;
   }
   return self;
 }
 
 - (void)executeOnMainWindow {
   UIWindow* mainWindow = [[UIApplication sharedApplication] keyWindow];
   DCHECK(mainWindow);
   [mainWindow chromeExecuteCommand:self];
 }
 
 @end