Use precompiled headers on Windows only when a flag is set.

build/common.gypi

 # Copyright (c) 2011 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 195 matching lines @@
 
       # Webrtc compilation is enabled by default. Set to 0 to disable.
       'enable_webrtc%': 1,
 
       # XInput2 multitouch support is disabled by default (use_xi2_mt=0).
       # Setting to non-zero value enables XI2 MT. When XI2 MT is enabled,
       # the input value also defines the required XI2 minor minimum version.
       # For example, use_xi2_mt=2 means XI2.2 or above version is required.
       'use_xi2_mt%': 0,
 
+      # Use of precompiled headers on Windows is off by default
+      # because of complications that it can cause with our
+      # infrastructure (trybots etc.).  Enable by setting to 1 in
+      # ~/.gyp/include.gypi or via the GYP command line for ~20-25%
+      # faster builds.
+      'win_use_pch%': 0,

[M-A Ruel, 2011/09/26 16:04:27]

I find it sad that chromium_mac_pch and win_use_pch. IMHO there is no need to
specify the OS there, it's unambiguously a flag to enable it if possible.

[Mark Mentovai, 2011/09/26 16:10:59]

Marc-Antoine Ruel wrote:
The Mac buildbots may have this set in unexpected places, like in
~/.gyp/include.gypi. If you change the name used on the Mac, you’ll need to
track down these locations and fix them.

+
       'conditions': [
         # TODO(epoger): Figure out how to set use_skia=1 for Mac outside of
         # the 'conditions' clause.  Initial attempts resulted in chromium and
         # webkit disagreeing on its setting.
         ['OS=="mac"', {
           'use_skia%': 1,
           # Mac uses clang by default, so turn on the plugin as well.
           'clang_use_chrome_plugins%': 1,
         }, {
           'use_skia%': 1,
@@ skipping 103 matching lines @@
     'armv7%': '<(armv7)',
     'arm_neon%': '<(arm_neon)',
     'sysroot%': '<(sysroot)',
     'disable_sse2%': '<(disable_sse2)',
     'component%': '<(component)',
     'use_titlecase_in_grd_files%': '<(use_titlecase_in_grd_files)',
     'use_third_party_translations%': '<(use_third_party_translations)',
     'remoting%': '<(remoting)',
     'use_threaded_compositing%': '<(use_threaded_compositing)',
     'enable_webrtc%': '<(enable_webrtc)',
+    'win_use_pch%': '<(win_use_pch)',
     'p2p_apis%': '<(p2p_apis)',
     'configuration_policy%': '<(configuration_policy)',
     'safe_browsing%': '<(safe_browsing)',
     'clang_use_chrome_plugins%': '<(clang_use_chrome_plugins)',
     'asan%': '<(asan)',
     'enable_register_protocol_handler%': '<(enable_register_protocol_handler)',
     'enable_smooth_scrolling%': '<(enable_smooth_scrolling)',
     'enable_web_intents%': '<(enable_web_intents)',
     # Whether to build for Wayland display server
     'use_wayland%': 0,
@@ skipping 274 matching lines @@
         # Choose static link by build type.
         'conditions': [
           ['android_build_type==0', {
             'static_link_system_icu%': 1,
           }, {
             'static_link_system_icu%': 0,
           }],
         ],
         # Enable to use system sqlite.
         'use_system_sqlite%': '<(android_build_type)',
-        # Enable to use system libjpeg. 
+        # Enable to use system libjpeg.
         'use_system_libjpeg%': '<(android_build_type)',
         # Enable to use the system libexpat.
         'use_system_libexpat%': '<(android_build_type)',
         # Enable to use the system stlport, otherwise statically
         # link the NDK one?
         'use_system_stlport%': '<(android_build_type)',
         # Copy it out one scope.
         'android_build_type%': '<(android_build_type)',
       }],  # OS=="android"
       ['OS=="mac"', {
@@ skipping 1786 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }