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

third_party/brotli/BUILD.gn

 # Copyright 2014 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.
 
 source_set("brotli") {
   sources = [
     "dec/bit_reader.c",
     "dec/bit_reader.h",
     "dec/context.h",
     "dec/decode.c",
     "dec/decode.h",
     "dec/dictionary.h",
     "dec/huffman.c",
     "dec/huffman.h",
     "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",
   ]
 
+  config("brotli_warnings") {
+    if (is_clang) {
+      # 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.)
+      cflags = [ "-Wno-unused-function" ]
+    }
+  }
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
+  configs += [ ":brotli_warnings" ]
 
   # Since we are never debug brotli, freeze the optimizations to -O2.
   if (is_debug) {
     configs -= [ "//build/config/compiler:no_optimize" ]
   } else {
     configs -= [ "//build/config/compiler:optimize" ]
   }
   configs += [ "//build/config/compiler:optimize_max" ]
 
   include_dirs = [ "dec" ]
 }