This is a reland of 9d34fa0c

TBR=ishell@chromium.org

Original change's description:
> [parser] Optimize directive parsing especially for preparser
>
> - Avoid allocating AstRawString in the preparser
> - Use fast LiteralEquals to compare the directive.
>
> Bug: chromium:901250
> Change-Id: I178aca812f6c0ffa28d7f48b707316a5a99a2ac0
> Reviewed-on: https://chromium-review.googlesource.com/c/1314570
> Commit-Queue: Toon Verwaest <verwaest@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#57217}

Bug: chromium:901250
Change-Id: I01dfd882923d3f37a08ca0be193474d38e273927
Reviewed-on: https://chromium-review.googlesource.com/c/1314578Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57220}