Add precompiled headers to GN build for large targets.

ash/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/features.gni")
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 
 gypi_values = exec_script("//build/gypi_to_gn.py",
                           [ rebase_path("ash.gyp") ],
                           "scope",
                           [ "ash.gyp" ])
 
 component("ash") {
   sources = gypi_values.ash_sources
 
+  configs += [ "//build/config:precompiled_headers" ]
   defines = [ "ASH_IMPLEMENTATION" ]
 
   public_deps = [
     "//ash/resources",
     "//ash/strings",
   ]
   deps = [
     "//base",
     "//base:i18n",
     "//base/third_party/dynamic_annotations",
@@ skipping 139 matching lines @@
     "//ui/gfx/geometry",
     "//ui/resources",
     "//ui/strings",
     "//ui/views",
     "//ui/views/controls/webview",
     "//ui/web_dialogs",
     "//url",
   ]
 }
 
-static_library("test_support") {
+source_set("test_support") {
   testonly = true
   sources = gypi_values.ash_test_support_sources
+  configs += [ "//build/config:precompiled_headers" ]
 
   public_deps = [
     ":ash",
   ]
   deps = [
     "//ash/resources",
     "//content/test:test_support",
     "//skia",
     "//testing/gtest",
     "//ui/accessibility",
@@ skipping 55 matching lines @@
     "//ui/resources",
     "//ui/views",
     "//ui/views:test_support",
     "//ui/views/examples:views_examples_lib",
     "//ui/views/examples:views_examples_with_content_lib",
   ]
 }
 
 test("ash_unittests") {
   sources = gypi_values.ash_unittests_sources
+  configs += [ "//build/config:precompiled_headers" ]
 
   deps = [
     ":ash",
     ":ash_with_content",
     ":test_support",
     "//ash/resources",
     "//ash/strings",
     "//base",
     "//base/allocator",
     "//base/test:test_support",
@@ skipping 128 matching lines @@
     "//testing/gtest",
     "//ui/accessibility",
   ]
 
   if (is_chromeos) {
     deps += [ "//ui/display" ]
   }
 }
 # When adding support for isolates, please have a look at run-time dependencies
 # in the ash_unittests_run target in ash.gyp.