This follows the "CRTP" pattern used elsewhere in the Parser rather than
a branch on IsPreParser(). Also merge GetUnexpectedTokenMessage()
into ReportUnexpectedTokenAt().

Change-Id: I8eaa5cc3230c4660624a48c705f80d1a60a2710b
Reviewed-on: https://chromium-review.googlesource.com/c/1423094Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59006}