Add precompiled headers to GN build for large targets.

ppapi/thunk/BUILD.gn

 # Copyright 2015 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("thunk") {
   # In GYP this is the same target as shared_impl. In GN these are split apart
   # for clarity but to get component builds correct, targets must only depend
   # on these via the shared_impl component.
   # TODO(brettw) separate these when GYP compat is no longer required.
   visibility = [ "//ppapi/shared_impl" ]
@@ skipping 170 matching lines @@
     sources += [
       "ppb_graphics_3d_thunk.cc",
       "ppb_host_resolver_private_thunk.cc",
       "ppb_tcp_server_socket_private_thunk.cc",
       "ppb_tcp_socket_private_thunk.cc",
       "ppb_udp_socket_private_thunk.cc",
       "ppb_x509_certificate_private_thunk.cc",
     ]
   }
 
+  configs += [ "//build/config:precompiled_headers" ]
   defines = [
     # This target goes in the same library as shared_impl (in GYP they are the
     # same).
     "PPAPI_SHARED_IMPLEMENTATION",
 
     "PPAPI_THUNK_IMPLEMENTATION",
   ]
 }