This switches the parser to use token positions (i.e. {Position})
instead of stream positions (i.e. {GetPosition}) everywhere. Access to
the latter is being removed as it is unsupported when the scanner is in
rewind state anyways. This prevents "skipping" a token when seeking.

R=bradnelson@chromium.org
BUG=v8:6127

Change-Id: I9c13dd20a981061a2bccc4fb57e5c57d2a64ac5c
Reviewed-on: https://chromium-review.googlesource.com/480300Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44699}