drover: Use Python style for accessing last item in array.

drover.py

 #!/usr/bin/env python
 # Copyright (c) 2011 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 optparse
 import os
 import re
 import string
 import sys
@@ skipping 386 matching lines @@
     # Branch number is the third value in the quad.
     branch_dict[version[2]] = major
 
   if not branch_dict:
     # |milestone| not found.
     print "Milestone provided is invalid"
     return None
 
   # The following returns a sorted list of the keys of |branch_dict|.
   sorted_branches = sorted(branch_dict)
-  branch = sorted_branches[len(sorted_branches) - 1]
+  branch = sorted_branches[-1]
 
   # If all keys match, the branch is the same for all platforms given
   # |milestone|. This is the safe case, so return the branch.
   if len(sorted_branches) == 1:
     return branch
 
   # Not all of the platforms have the same branch. Prompt the user and return
   # the greatest (by value) branch on success.
   if prompt("Not all platforms have the same branch number, "
             "continue with branch %s?" % branch):
@@ skipping 210 matching lines @@
 
   if options.branch and options.milestone:
     option_parser.error("--branch cannot be used with --milestone")
     return 1
 
   return drover(options, args)
 
 
 if __name__ == "__main__":
   sys.exit(main())