Fix bug in BackwardsCharacterIterator::endPosition introduced in latest change.

TBR=yosin@chromium.org
BUG=388681

Review URL: https://codereview.chromium.org/560653003

git-svn-id: svn://svn.chromium.org/blink/trunk@181827 bbb929c8-8fbe-4397-9dbb-9b2b20218538