Add precompiled headers to GN build for large targets.

content/child/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/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//content/child/child.gni")
 
 source_set("child") {
   # Only the public target should depend on this. All other targets (even
   # internal content ones) should depend on the public one.
   visibility = [ "//content/public/child:child_sources" ]
 
   sources = rebase_path(content_child_gypi_values.private_child_sources,
                         ".",
                         "//content")
 
+  configs += [ "//build/config:precompiled_headers" ]
+
   public_deps = [
     "//third_party/mojo/src/mojo/edk/system",
   ]
 
   deps = [
     "//base",
     "//components/mime_util",
     "//components/tracing",
     "//components/tracing:startup_tracing",
     "//components/webcrypto",
@@ skipping 97 matching lines @@
 
   if (use_aura && is_mac) {
     # This file is already excluded on non-Mac.
     sources -= [ "npapi/webplugin_delegate_impl_mac.mm" ]
   }
 
   if (is_win || !use_aura) {
     sources -= [ "npapi/webplugin_delegate_impl_aura.cc" ]
   }
 }