First step for cacheing the result of glGetString calls is mostly
cleanup.

Bug: chromium:1173672
Change-Id: I47281a09b9bd5859655d16376977a6eabcfbb3c4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2676203
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>