Add precompiled headers to GN build for large targets.

extensions/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.
 
 # TODO(rockot) bug 505926: Don't include chrome files from here.
 # See chrome_browser_tests_extensions_sources below
 import("//chrome/chrome_tests.gni")
 import("//extensions/extensions.gni")
 import("//testing/test.gni")
 import("//tools/grit/grit_rule.gni")
@@ skipping 130 matching lines @@
       rebase_path(extensions_tests_gypi_values.extensions_unittests_sources,
                   ".",
                   "//extensions")
 
   sources += [
     # TODO(rockot): DisplayInfoProvider::Create() is only implemented in Chrome
     # and app_shell. This is wrong.
     "shell/browser/shell_display_info_provider.cc",
   ]
 
+  configs += [ "//build/config:precompiled_headers" ]
+
   deps = [
     ":extensions_resources",
     ":shell_and_test_pak",
     ":test_support",
     "//base",
     "//base:prefs_test_support",
     "//base/test:test_support",
     "//components/keyed_service/content",
     "//components/user_prefs",
     "//content/test:test_support",
@@ skipping 171 matching lines @@
     "//testing/gtest",
     "//third_party/hunspell",
     "//third_party/icu",
     "//third_party/libpng",
     "//third_party/zlib",
     "//ui/base:test_support",
     "//ui/resources:ui_test_pak",
     "//ui/web_dialogs:test_support",
   ]
 }