Refactor the check for keeping shadow data into a feature so it's only
initialized once.

Bug: angleproject:3020
Change-Id: I45575c246afa7cd54e3a07d7a8464f4d4f45b3be
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1769064
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>