Most scanner logic doesn't need to care about surrogate pairs, so we can just
push it down to identifier scanning.

This CL additionally drops some explicit kEndOfInput checks that are subsumed
by predicates seemlessly returning false for kEndOfInput (-1).

Change-Id: If031a9355ab5fbca0c3b647045e3034f42923979
Reviewed-on: https://chromium-review.googlesource.com/1124447Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54172}