- 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}