Now that identifiers in the preparser also carry their string, we can simply
check that rather than relying on a weird "keyword". Dropping __proto__ as a
keyword allows us to delist '_' as keyword character.

Change-Id: I775df25f77a84de92a60790ca665f16d52abf4bf
Reviewed-on: https://chromium-review.googlesource.com/c/1329692
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57427}