Fix _check_upward_for_depot_tools in webkit_finder.py.

Simple fix in function that, as it seems, wasn't called a single time.

BUG=

Review URL: https://chromiumcodereview.appspot.com/24123004

git-svn-id: svn://svn.chromium.org/blink/trunk@157886 bbb929c8-8fbe-4397-9dbb-9b2b20218538