AtomicString and StringBuilder should be bloat-free

There are no longer any callers of bloatedCharacters for StringBuilder and
AtomicString. We can remove their bloatedCharacters API.

R=eseidel@chromium.org
BUG=254708

Review URL: https://codereview.chromium.org/19310002

git-svn-id: svn://svn.chromium.org/blink/trunk@154275 bbb929c8-8fbe-4397-9dbb-9b2b20218538