Inputs to JSON can be two-byte because payload strings can contain two-byte
characters, without actually having any one-byte property key. Rather than
eagerly converting the string to one-byte, we can perform a string-table lookup
with a two-byte string key; only converting the result to one-byte if it's a
new key.

This speeds up json parsing of two-byte json from the Youtube benchmark by 20%.

Change-Id: If6d4a37d331724f48b008aef8ec3e28d366cd038
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1619866
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61680}