The other backend description strings are in DisplayImpl. This will
help with caching the result of glGetString in the GL backend. Also
Update the getters to not be const in order to allow caching.

Bug: chromium:1173672
Change-Id: I43df35688762b23429f47f169c04482cf4cd089a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2676881Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>