Rework build/mac/tweak_info_plist.py to not rely on environment variables.

chrome/app_shim/app_shim.gypi

 # Copyright 2014 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.
 
 {
   'targets': [
     {
       # This is the part of the shim process compiled into Chrome. It runs only
       # in the shim process, after the shim finds and loads the Chrome
       # Framework bundle.
@@ skipping 56 matching lines @@
       },
       'postbuilds' : [
         {
           # Modify the Info.plist as needed.  The script explains why this
           # is needed.  This is also done in the chrome and chrome_dll
           # targets.  In this case, --breakpad=0, --keystone=0, and --scm=0
           # are used because Breakpad, Keystone, and SCM keys are
           # never placed into the app mode loader.
           'postbuild_name': 'Tweak Info.plist',
           'action': ['<(tweak_info_plist_path)',
+                     '--plist=${TARGET_BUILD_DIR}/${INFOPLIST_PATH}',
                      '--breakpad=0',
                      '--keystone=0',
                      '--scm=0'],
         },
       ],
     },  # target app_mode_app
   ],  # targets
 }