Add precompiled headers to GN build for large targets.

build/config/BUILD.gn

 # Copyright (c) 2013 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/allocator.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
@@ skipping 400 matching lines @@
 # Add this config to your target to enable precompiled headers.
 #
 # On Windows, precompiled headers are done on a per-target basis. If you have
 # just a couple of files, the time it takes to precompile (~2 seconds) can
 # actually be longer than the time saved. On a Z620, a 100 file target compiles
 # about 2 seconds faster with precompiled headers, with greater savings for
 # larger targets.
 #
 # Recommend precompiled headers for targets with more than 50 .cc files.
 config("precompiled_headers") {
-  # TODO(brettw) enable this when GN support in the binary has been rolled.
-  #if (is_win) {
-  if (false) {
+  if (is_win) {
     # This is a string rather than a file GN knows about. It has to match
     # exactly what's in the /FI flag below, and what might appear in the source
     # code in quotes for an #include directive.
     precompiled_header = "build/precompile.h"
 
     # This is a file that GN will compile with the above header. It will be
     # implicitly added to the sources (potentially multiple times, with one
     # variant for each language used in the target).
     precompiled_source = "//build/precompile.cc"
 
     # Force include the header.
     cflags = [ "/FI$precompiled_header" ]
   }
 }