Use PCH by default in VS 2010.

build/common.gypi

 # Copyright (c) 2012 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 233 matching lines @@
       # thread. Set to 1 to turn on experimental support for out-of-process
       # plugins to make call of the main thread.
       'enable_pepper_threading%': 0,
 
       # 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.
+      # Use of precompiled headers on Windows.
+      #
+      # This is on by default in VS 2010, but off by default for VS
+      # 2008 because of complications that it can cause with our
+      # trybots etc.
+      #
+      # This variable may be explicitly set to 1 (enabled) or 0
+      # (disabled) in ~/.gyp/include.gypi or via the GYP command line.
+      # This setting will override the default.
+      #
+      # Note that a setting of 1 is probably suitable for most or all
+      # Windows developers using VS 2008, since precompiled headers
+      # provide a build speedup of 20-25%.  There are a couple of
+      # small workarounds you may need to use when using VS 2008 (but
+      # not 2010), see
+      # http://code.google.com/p/chromium/wiki/WindowsPrecompiledHeaders
+      # for details.
       'chromium_win_pch%': 0,
 
       '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%': '<(use_skia_on_mac)',
           # Mac uses clang by default, so turn on the plugin as well.
           'clang_use_chrome_plugins%': 1,
@@ skipping 89 matching lines @@
           'enable_webrtc%': 0,
         }],
 
         # Use GPU accelerated cross process image transport by default
         # on linux builds with the Aura window manager
         ['views_compositor==1 and OS=="linux"', {
           'ui_compositor_image_transport%': 1,
         }, {
           'ui_compositor_image_transport%': 0,
         }],
+
+        # Turn precompiled headers on by default for VS 2010.
+        ['OS=="win" and MSVS_VERSION=="2010"', {
+          'chromium_win_pch%': 1
+        }],
       ],
     },
 
     # Copy conditionally-set variables out one scope.
     'branding%': '<(branding)',
     'buildtype%': '<(buildtype)',
     'target_arch%': '<(target_arch)',
     'host_arch%': '<(host_arch)',
     'library%': 'static_library',
     'aura_show_about_flag_window_mode%': '<(aura_show_about_flag_window_mode)',
@@ skipping 2423 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',
   },
 }