Revert "Remove StaticString ASSERTs"
and "Remove the pointer from StringImpl"

This reverts commit 2352bfc2026feaeb666f5226c818bd3e803027ae.
and bea03164a7aebe29d05278fa5e8b881a60acbc49.

The tests still crash on Windows debug. Sadness.

git-svn-id: svn://svn.chromium.org/blink/trunk@155418 bbb929c8-8fbe-4397-9dbb-9b2b20218538