This way we can avoid reencoding everything to utf16 (buffered) and avoid the
overhead of needing to check the encoding for each character individually.

This may result in a minor asm.js scanning regression due to one-byte tokens
possibly being more common.

Change-Id: I90b51c256d56d4f4fa2d235d7e1e58fc01e43f31
Reviewed-on: https://chromium-review.googlesource.com/1172437
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55217}