Make content_shell link in the GN build.

third_party/mesa/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.
 
 import("//build/config/ui.gni")
 
 config("mesa_headers_config") {
   include_dirs = [ "src/include" ]
   if (use_x11) {
     defines = [ "MESA_EGL_NO_X11_HEADERS" ]
@@ skipping 216 matching lines @@
   ]
 
   configs -= [
     "//build/config/compiler:chromium_code",
   ]
   configs += [
     ":mesa_internal_config",
     "//build/config/compiler:no_chromium_code",
   ]
 
+  if (is_clang) {
+    # Mesa triggers some of these Clang warnings.
+    configs -= [ "//build/config/clang:extra_warnings" ]
+  }
+
   deps = [
     ":mesa_headers",
   ]
 }
 
 static_library("mesa") {
   sources = [
     "$generated_src_dir/mesa/builtin_function.cpp",
     "$generated_src_dir/mesa/glapi_mapi_tmp_shared.h",
     "src/src/mapi/mapi/entry.c",
@@ skipping 425 matching lines @@
   deps = [
     ":mesa_headers",
     ":mesa",
     ":mesa_libglslcommon",
   ]
 }
 
 }  # !is_android
 
 # TODO(GYP) Android osmesa_in_lib_dir target.