Add a gyp flag to allow use of the system's version of libexpat

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 924 matching lines @@
 
         # Choose static link by build type.
         'conditions': [
           ['android_build_type==0', {
             'static_link_system_icu%': 1,
           }, {
             'static_link_system_icu%': 0,
           }],
         ],
         # TODO(steveblock): Investigate using the system versions of sqlite,
-        # libjpeg, expat and stlport.
+        # libjpeg and stlport.
         # Enable to use system sqlite.
         'use_system_sqlite%': 0,  # '<(android_build_type)',
         # Enable to use system libjpeg.
         'use_system_libjpeg%': 0,  # '<(android_build_type)',
-        # Enable to use the system libexpat.
-        'use_system_libexpat%': 0,  # '<(android_build_type)',
+        # Enable to use the system expat.
+        'use_system_expat%': '<(android_build_type)',

[Lei Zhang, 2012/07/09 23:05:33]

Is |android_build_type| ever going to be non-zero? I don't know the future plans
for this variable.

         # Enable to use the system stlport, otherwise statically
         # link the NDK one?
         'use_system_stlport%': 0,  # '<(android_build_type)',
         # Copy it out one scope.
         'android_build_type%': '<(android_build_type)',
       }],  # OS=="android"
       ['OS=="mac"', {
         # Enable clang on mac by default!
         'clang%': 1,
         # Compile in Breakpad support by default so that it can be
@@ skipping 2274 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',
   },
 }