Fix wrong index in process_file_template function.

BUG= none
TEST= process_file_template(['a', 'b'], '{{source}}')
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/124563002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243036 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243036