Enforce marking "override" for functions overriding Blink.

build/common.gypi

 # Copyright (c) 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 2209 matching lines @@
                   }],
                 ],
               },
               'clang_dynlib_flags%': '-Xclang -load -Xclang <(clang_lib_path) ',
             }, { # OS == "win"
               # On Windows, the plugin is built directly into clang, so there's
               # no need to load it dynamically.
               'clang_dynlib_flags%': '',
             }],
           ],
-          'clang_plugin_args%': '-Xclang -plugin-arg-find-bad-constructs -Xclang check-templates ',
+          'clang_plugin_args%': '-Xclang -plugin-arg-find-bad-constructs -Xclang check-templates '
+          '-Xclang -plugin-arg-find-bad-constructs -Xclang enforce-overriding-blink ',
         },
         # If you change these, also change build/config/clang/BUILD.gn.
         'clang_chrome_plugins_flags%':
           '<(clang_dynlib_flags)'
           '-Xclang -add-plugin -Xclang find-bad-constructs <(clang_plugin_args)',
       }],
       ['asan==1 or msan==1 or lsan==1 or tsan==1', {
         'clang%': 1,
         'use_allocator%': 'none',
         'use_sanitizer_options%': 1,
@@ skipping 4132 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }