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/+/2587451Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Stephen White <senorblanco@chromium.org>