Add support for libxml as a system library target on iOS

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 843 matching lines @@
           }],
           # All Chrome builds have breakpad symbols, but only process the
           # symbols from official builds.
           ['(branding=="Chrome" and buildtype=="Official")', {
             'linux_dump_symbols%': 1,
           }],
         ],
       }],  # os_posix==1 and OS!="mac" and OS!="ios"
       ['OS=="ios"', {
         'disable_nacl%': 1,
+        'use_system_libxml%': 1,
       }],
       ['OS=="android"', {
         # Location of Android NDK.
         'variables': {
           'variables': {
             'android_ndk_root%': '<!(/bin/echo -n $ANDROID_NDK_ROOT)',
             # Android uses x86 instead of ia32 for their target_arch
             # designation.
             # TODO(wistoch): Adjust the target_arch naming scheme to avoid
             # confusion.
@@ skipping 2416 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',
   },
 }