Add "reduced shader" testing mode.

Currently doesn't do anything other than add new configs that set
a GrContextOption that becomes available in GrCaps. Runs new configs
on Perf/Test bots (MTL/iPhone11 and GL/Ubuntu).

Bug: skia:11844
Change-Id: I58586cae0980e52701abd1633dbc79b381f6015b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/394996
Reviewed-by: Eric Boren <borenet@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
12 files changed