Fix handling of ast.Str object.

TBR=iannucci@chromium.org
BUG=

Review URL: https://codereview.chromium.org/686183003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@292733 0039d316-1c4b-4281-b951-d872f2087c98

roll_dep.py

@@ -324,7 +324,7 @@ def update_deps(deps_file, dep_path, dep_name, new_rev, comment):
         else:
           update_deps_entry(deps_lines, deps_ast, value_node, new_rev, comment)
           commit_msg = generate_commit_message(
-              deps_locals['deps_os'][os_name], dep_path, dep_name, new_rev)
+              deps_locals['deps_os'][os_name.s], dep_path, dep_name, new_rev)
   if commit_msg:
     print 'Pinning %s' % dep_name
     print 'to revision %s' % new_rev