Track *all* external strings in the external string table.

Up till now, external strings may be tracked in the string table
(for internalized strings) or the external string table, depending
on in which order internalize and externalize happened.

The internalized string table should not have to deal with external
strings, all of which should be tracked by the external string table.

R=svenpanne@chromium.org
BUG=326489
LOG=N

Review URL: https://codereview.chromium.org/103663006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18285 ce2b1a6d-e550-0410-aec6-3dcde31c8c00