[run_web_tests] Add option to run virtual tests in parallel

This reduced the local runtime of virtual/layout_ng_flex_box from 2m22s
down to 0m18s. I had previously been jumping through hoops locally to
not run the virtual suite at all.

Change-Id: Idf4ba1c6cf6d7fc43d5873b873c7eb8be313e474
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1999185
Commit-Queue: David Grogan <dgrogan@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731765}
3 files changed