V8 passes the command explicitly to each swarming task, hence it's
not necessary to store the command in the isolate.

This drops the Chromium-specific code in MB that creates the
swarming command based on Chromium test features.

This also makes the swarming targets option a no-op to allow
activating it on the infra side without disruption.

Bug: chromium:669910
Change-Id: I6cb03f05d034092a25d879d52b4d64952493f55b
Reviewed-on: https://chromium-review.googlesource.com/779148Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49520}