TextIterator should not use bloatedCharacters()

This CL refactors TextIterator to avoid calling bloatedCharacters on Strings.
It still calls bloatedCharacters on RenderText, but I have a plan to fix that
in a future CL.

R=eseidel
BUG=254708

Review URL: https://chromiumcodereview.appspot.com/19752002

git-svn-id: svn://svn.chromium.org/blink/trunk@154525 bbb929c8-8fbe-4397-9dbb-9b2b20218538