With 32-bit kTaggedSize small strings may be not externalizable.

Bug: v8:7703
Change-Id: I34002568214742dadb2358fca97dfb4b92a5342a
Reviewed-on: https://chromium-review.googlesource.com/c/1480373Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59770}