Re-enable precompiled headers in GN 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/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//build/module_args/v8.gni")
+import("//build/toolchain/goma.gni")
 
 declare_args() {
   # When set, turns off the (normally-on) iterator debugging and related stuff
   # that is normally turned on for Debug builds. These are generally useful for
   # catching bugs but in some cases may cause conflicts or excessive slowness.
   disable_iterator_debugging = false
 
   # Set to true to not store any build metadata, e.g. ifdef out all __DATE__
   # and __TIME__. Set to 0 to reenable the use of these macros in the code
   # base. See http://crbug.com/314403.
@@ skipping 389 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 everything is working. A previous attempt
-  # to enable this caused some bot redness which needs ot be investigated.
-  #if (is_win) {
-  if (false) {
+  if (is_win && !is_official_build && !use_goma) {
     # 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.
+    cflags_c = [ "/wd4206" ]
   }
 }