rev_sdk.py: Don't "git rm" nonexistent directories in Chromium.

mojo/tools/roll/rev_sdk.py

 #!/usr/bin/env python
 # 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.
 
 import os
 import sys
 from utils import commit
 from utils import mojo_root_dir
 from utils import system
 
 dirs_to_clone = [
   "mojo/edk",
   "mojo/public",
   "mojo/services/public",
   "mojo/services/surfaces/public",
   "mojo/services/view_manager/public",
   "mojo/services/window_manager/public",
 ]
 
 def rev(source_dir, chromium_dir):
   src_commit = system(["git", "show-ref", "HEAD", "-s"], cwd=source_dir).strip()
 
   for d in dirs_to_clone:
-    print "removing directory %s" % d
-    system(["git", "rm", "-r", d], cwd=chromium_dir)
+    if os.path.exists(os.path.join(chromium_dir, d)):
+      print "removing directory %s" % d
+      system(["git", "rm", "-r", d], cwd=chromium_dir)
     print "cloning directory %s" % d
     files = system(["git", "ls-files", d], cwd=source_dir)
     for f in files.splitlines():
       # Don't copy presubmit files over since the code is read-only on the
       # chromium side.
       if os.path.basename(f) == "PRESUBMIT.py":
         continue
       dest_path = os.path.join(chromium_dir, f)
       system(["mkdir", "-p", os.path.dirname(dest_path)])
       system(["cp", os.path.join(source_dir, f), dest_path])
     os.chdir(chromium_dir)
     system(["git", "add", d], cwd=chromium_dir)
 
   with open("mojo/public/VERSION", "w") as version_file:
     version_file.write(src_commit)
   system(["git", "add", "mojo/public/VERSION"], cwd=chromium_dir)
   commit("Update mojo sdk to rev " + src_commit, cwd=chromium_dir)
 
 if len(sys.argv) != 2:
   print "usage: rev_sdk.py <chromium source dir>"
   sys.exit(1)
 
 rev(mojo_root_dir, sys.argv[1])