Introduce String and AtomicString size check

SSIA. Considering the memory sensitiveness of strings in Blink
we should make sure that no change can accidentally increase
their size.

Review URL: https://codereview.chromium.org/205583005

git-svn-id: svn://svn.chromium.org/blink/trunk@169750 bbb929c8-8fbe-4397-9dbb-9b2b20218538