Enforce marking "override" for functions overriding Blink.

build/config/clang/BUILD.gn

 # Copyright (c) 2013 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("clang.gni")
 
 config("find_bad_constructs") {
   if (clang_use_chrome_plugins) {
     cflags = []
 
@@ skipping 22 matching lines @@
 
     cflags += [
       "-Xclang",
       "-add-plugin",
       "-Xclang",
       "find-bad-constructs",
       "-Xclang",
       "-plugin-arg-find-bad-constructs",
       "-Xclang",
       "check-templates",
+      "-Xclang",
+      "-plugin-arg-find-bad-constructs",
+      "-Xclang",
+      "enforce-overriding-blink",
     ]
   }
 }
 
 # Enables some extra Clang-specific warnings. Some third-party code won't
 # compile with these so may want to remove this config.
 config("extra_warnings") {
   cflags = [
     "-Wheader-hygiene",
 
     # Warns when a const char[] is converted to bool.
     "-Wstring-conversion",
   ]
 }