Encasulate StringImpl allocation size calculation within a single function

SSIA.

Inspired by WebKit r163326 from Anders Carlsson <andersca@apple.com>.

Review URL: https://codereview.chromium.org/129863006

git-svn-id: svn://svn.chromium.org/blink/trunk@166489 bbb929c8-8fbe-4397-9dbb-9b2b20218538