This is 3-4x faster than using the Utf8Decoder. This matters for proper
parse-time measurements using d8.

Change-Id: I9870e9fbe400ec022a6eeb20491c80a2a32f8519
Reviewed-on: https://chromium-review.googlesource.com/c/1451827
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59347}