Mark TextIterator::characters as bloatedCharacters

TextIterator secretly lets its clients bloat characters.

R=eseidel@chromium.org
BUG=254708

Review URL: https://codereview.chromium.org/19697002

git-svn-id: svn://svn.chromium.org/blink/trunk@154436 bbb929c8-8fbe-4397-9dbb-9b2b20218538