Explicitly pass -Wno-unused-function to the targets that need it.

third_party/brotli/brotli.gyp

 # Copyright 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.
 
 {
   'targets': [
     {
       'target_name': 'brotli',
       'type': 'static_library',
       'include_dirs': [
@@ skipping 11 matching lines @@
         'dec/prefix.h',
         'dec/safe_malloc.c',
         'dec/safe_malloc.h',
         'dec/state.c',
         'dec/state.h',
         'dec/streams.c',
         'dec/streams.h',
         'dec/transform.h',
         'dec/types.h',
       ],
+      'variables': {
+        'clang_warning_flags': [
+          # IncrementalCopyFastPath in decode.c can be unused.
+          # (The file looks very different upstream, this is probably no longer
+          # needed after rolling brotli the next time.)
+          '-Wno-unused-function',
+        ],
+      },
       'conditions': [
         ['os_posix==1 and (target_arch=="arm" or target_arch=="armv7" or target_arch=="arm64")', {
           'cflags!': ['-Os'],
           'cflags': ['-O2'],
         }],
       ],
     },
   ],
 }