Immutable command class with shell, flags and
environment.

Command creation moved from worker to the main
process. Because of that there is no need to send
test cases beyond process boundaries and load test
suites in worker processes.

Bug: v8:6917
Change-Id: Ib6a44278095b4f7141eb9b96802fe3e8117678a6
Reviewed-on: https://chromium-review.googlesource.com/791710
Commit-Queue: Michał Majewski <majeski@google.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49746}