We had a way to do string to double without allocation that we were
using on StringData. Reuse that on StringRef for Strings that can
access the heap.

BUg: v8:7790
Change-Id: I30e6dace3fbf05eb8672ff1bad46f6c6d6fe1d6d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2450013Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70384}