Add kHeaderSize constant to SeqString.

Add kHeaderSize constant to SeqString.

This prevents potential misuse of SeqString::kHeaderSize as in the
case of live byte counting in incremental marking stub. All stubs
picked up the undefined size constant SeqString::kHeaderSize, thus
the computed size of all strings was off by two pointers slots.

R=lrn@chromium.org
BUG=v8:1672
TEST=mjsunit/object-seal.js,...

Review URL: http://codereview.chromium.org/7971009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9349 ce2b1a6d-e550-0410-aec6-3dcde31c8c00