We never mutate these tables; marking them const allows the loader to
shuffle them to a place that makes them readonly after they're init'ed.

This isn't intended to be a functional change.

Bug: chromium:1084580
Change-Id: If3356f6082945ef1109bf08aab94f7bfb606b951
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2216295
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>