Make it possible to change libc++ library files used for android

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 1758 matching lines @@
 
         'android_ndk_include': '<(android_ndk_sysroot)/usr/include',
         'android_ndk_lib': '<(android_ndk_sysroot)/<(android_ndk_lib_dir)',
         'android_sdk_tools%': '<(android_sdk_tools)',
         'android_aapt_path%': '<(android_sdk_tools)/aapt',
         'android_sdk%': '<(android_sdk)',
         'android_sdk_jar%': '<(android_sdk)/android.jar',
 
         'android_libcpp_root': '<(android_libcpp_root)',
         'android_libcpp_include': '<(android_libcpp_root)/libcxx/include',
-        'android_libcpp_libs_dir': '<(android_libcpp_root)/libs/<(android_app_abi)',
+        'android_libcpp_libs_dir%': '<(android_libcpp_root)/libs/<(android_app_abi)',
         'host_os%': '<(host_os)',
 
         # Location of the "objcopy" binary, used by both gyp and scripts.
         'android_objcopy%' : '<!(/bin/echo -n <(android_toolchain)/*-objcopy)',
 
         # Location of the "strip" binary, used by both gyp and scripts.
         'android_strip%' : '<!(/bin/echo -n <(android_toolchain)/*-strip)',
 
         # Location of the "readelf" binary.
         'android_readelf%' : '<!(/bin/echo -n <(android_toolchain)/*-readelf)',
@@ skipping 4397 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',
   },
 }