[parser] Introduce (peek_)end_position helpers

Change-Id: I5b5228427039ce31b9b978d406d860ff08b32dae
Reviewed-on: https://chromium-review.googlesource.com/1238543Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56154}