Make gn wrapper use the one in buildtools rather than tools/gn/bin.

gn.py

 #!/usr/bin/env python
 # Copyright 2013 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.
 
 """This script is a wrapper around the GN binary that is pulled from Google
 Cloud Storage when you sync Chrome. The binaries go into platform-specific
 subdirectories in the source tree.
 
 This script makes there be one place for forwarding to the correct platform's
 binary. It will also automatically try to find the gn binary when run inside
 the chrome source tree, so users can just type "gn" on the command line
 (normally depot_tools is on the path)."""
 
 import gclient_utils
 import os
 import subprocess
 import sys
 
 
-def RunGN(sourceroot):
-  # The binaries in platform-specific subdirectories in src/tools/gn/bin.
-  gnpath = os.path.join(sourceroot,
-                        'tools', 'gn', 'bin', gclient_utils.GetMacWinOrLinux(),
-                        'gn' + gclient_utils.GetExeSuffix())
-  return subprocess.call([gnpath] + sys.argv[1:])
-
-
 def main(args):
-  for arg in sys.argv:
-    if arg.startswith('--root='):
-      sourceroot = arg.replace('--root=', '')
-      dotfile_path = os.path.join(sourceroot, '.gn')
-      if not os.path.exists(dotfile_path):
-        print >> sys.stderr, 'gn.py: "%s" not found, exiting.' % dotfile_path
-        sys.exit(1)
-      return RunGN(sourceroot)
-
-  sourceroot = gclient_utils.FindFileUpwards('.gn')
-  if not sourceroot:
-    print >> sys.stderr, ('gn.py: No .gn file found in any parent of '
-                          'the current path.')
-    print >> sys.stderr, ('\nYou need to either be inside a checkout, '
-                          'or use --root to specify the checkout root.')
+  bin_path = gclient_utils.GetBuildtoolsPlatformBinaryPath()
+  if not bin_path:
+    print >> sys.stderr, ('gn.py: Could not find checkout in any parent of '
+                          'the current path.\nThis must be run inside a '
+                          'checkout.')
     sys.exit(1)
-  return RunGN(sourceroot)
+  gn_path = os.path.join(bin_path, 'gn' + gclient_utils.GetExeSuffix())
+  return subprocess.call([gn_path] + sys.argv[1:])
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))