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>