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Changes Dawn validation layer tests to run sequentially, rather than in
parallel jobs. This may address flaky failures being seen on Intel GPUs
when running with D3D12's GPU-based validation.

Bug: dawn:598
Change-Id: Ie53656f63bd2550d66f55f3f43f9c0baa19e1c91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2629610Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Brandon Jones <brandon1.jones@intel.com>
Cr-Commit-Position: refs/heads/master@{#845335}