A value of type OtherSeqString can change its type to OtherNonSeqString
via inplace internalization (and redirection via a ThinString). This can
lead to out of bounds memory accesses and generally correctness bugs, as
seen with crbug.com/822284.

This change might affect performance in some cases, and we'll need to
evaluate whether it's worth spending cycles on adding another mechanism
that leverages the sequential string information in a safe way on a case
by case basis.

Bug: chromium:822284
Change-Id: I0de77ec089a774236555f38c365f7548f454edfe
Reviewed-on: https://chromium-review.googlesource.com/966021Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51975}