This is a reland of 11c31e0a

I've landed upstream Skia change https://skia-review.googlesource.com/c/skia/+/351496
which should make this safe to reland.

Original change's description:
> Changes to build inside a Dawn checkout.
>
> Add an "angle_standalone" flag that can be disabled by Dawn.
> Put test definitions behind angle_standalone || build_with_chromium.
>
> Bug: angleproject:5462
> Change-Id: I58c9b18723384334156e2a3dd86ed3f89afcaade
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2587451
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Commit-Queue: Stephen White <senorblanco@chromium.org>

Bug: angleproject:5462
Change-Id: I4ff92d45b3692312a004f03b425234b78c5a6219
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2618040
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>