Instead of using a map type directly we abstract the info maps into an
encapsulated class. We can enforce a specific API set instead of using
the same API as the map class. This also cleans up a few of the APIs
related to these maps.

This change will allow future changes to the way the variables are
stored in the class without drastically changing the interface.

Bug: angleproject:3572
Bug: angleproject:4524
Change-Id: Ic1a63e1776c39f49b895a1274bae8282d7a6b9b5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2600080
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>