SwiftShader is required to implement WebGPU fallback adapters.
Perf tests are skipped for CPU adapters since they easily timeout
for intensive benchmarks.
Bug: chromium:1266550
Change-Id: Ib6e91da1128baae1770c797a69cf9ad605ea324d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/76421
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>