Remove String::characters()

This CL forces all new callers to use bloatedCharacters(), which will hopefully
make them feel bad and encourage them to use other APIs that don't bloat
memory.

R=eseidel
BUG=254708

Review URL: https://chromiumcodereview.appspot.com/18174012

git-svn-id: svn://svn.chromium.org/blink/trunk@153296 bbb929c8-8fbe-4397-9dbb-9b2b20218538