Enable precompiled headers for Blink on Windows.

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/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/features.gni")
+import("//build/config/pch.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//build/toolchain/goma.gni")
 
 # One common error that happens is that GYP-generated headers within gen/ get
 # included rather than the GN-generated ones within gen/ subdirectories.
 # TODO(GYP_GONE): Remove once GYP is gone (as well as exec_script exception).
 assert(
     exec_script("//build/dir_exists.py", [ "obj.host" ], "string") == "False",
     "GYP artifacts detected in $root_build_dir.$0x0A" +
         "You must wipe this directory before building with GN.")
 
 declare_args() {
   # When set (the default) enables C++ iterator debugging in debug builds.
   # Iterator debugging is always off in release builds (technically, this flag
   # affects the "debug" config, which is always available but applied by
   # default only in debug builds).
   #
   # Iterator debugging is generally useful for catching bugs. But it can
   # introduce extra locking to check the state of an iterator against the state
   # of the current object. For iterator- and thread-heavy code, this can
   # significantly slow execution.
   enable_iterator_debugging = true
-
-  # Normally we try to decide whether to use precompiled headers or
-  # not based on the other build arguments, but in some cases it is
-  # easiest to force them off explicitly.
-  disable_precompiled_headers = false
 }
 
 # ==============================================
 #   PLEASE DO NOT ADD MORE THINGS TO THIS LIST
 # ==============================================
 #
 # Legacy feature defines applied to all targets.
 #
 # These are applied to every single compile in the build and most of them are
 # only relevant to a few files. This bloats command lines and causes
@@ skipping 332 matching lines @@
 
 # Add this config to your target to enable precompiled headers.
 #
 # 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") {
-  if (!is_official_build && !use_goma && !disable_precompiled_headers) {
+  if (enable_precompiled_headers) {
     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" ]
 
       # Disable warning for "this file was empty after preprocessing". This
       # error is generated only in C mode for ANSI compatibility. It conflicts
       # with precompiled headers since the source file that's "compiled" for
       # making the precompiled header is empty.
       #
       # This error doesn't happen every time. In VS2013, it seems if the .pch
       # file doesn't exist, no error will be generated (probably MS tested this
       # case but forgot the other one?). To reproduce this error, do a build,
       # then delete the precompile.c.obj file, then build again.
+      #
+      # TODO(sof): determine VS2015 status and retire the setting from all
+      # precompiled configurations.
       cflags_c = [ "/wd4206" ]
     } else if (is_mac) {
       precompiled_header = "build/precompile.h"
       precompiled_source = "//build/precompile.h"
     }
   }
 }