Respect KHR_parallel_shader_compile for program link Parellel shader compilation was using the single-threaded vs multi-threaded pool appropriately, while program link was always being multi-threaded. This change makes sure the program link tasks uses the same pool as shader compilation per KHR_parallel_shader_compile. Bug: angleproject:8297 Change-Id: I0508617678a6e875fc0719a2d447cf1a9c5ca40f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4762086 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>