Use Vector as SSV's Writer buffer, to avoid mysterious realloc issue

A Blink@155225 changed SSV to serialize using StringBuffer instead of
Vector, but this seems to hit an issue with fastRealloc calls. Revert
back to Vector while we try and track down the root cause.

BUG=266837
R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23621033

git-svn-id: svn://svn.chromium.org/blink/trunk@157531 bbb929c8-8fbe-4397-9dbb-9b2b20218538