[snapshot] Use ThinStrings instead of custom forwarding using the hashfield to deserialize internalized strings

This is necessary to be able to store the string length next to the hash field in the same 64bit word on 64bit architectures. Otherwise the forwarding pointer will overwrite the length breaking heap guarantees. ThinStrings already just do the right thing, so just use that instead.

Bug: v8:7065
Change-Id: I4922365e72421995cd11437cb91572ff56f8a9e8
Reviewed-on: https://chromium-review.googlesource.com/763231Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49352}