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

Review URL: http://codereview.chromium.org/8883034

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@113720 0039d316-1c4b-4281-b951-d872f2087c98

drover.py

@@ -404,7 +404,7 @@ def getBranchForMilestone(milestone):
 
   # 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.