This moves code for running d8 into its own class. No functional
changes intended.

No-Try: true
Bug: chromium:1023091
Change-Id: I7cbfeebd2911dc758322f89cf93666550f2956d9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1906378
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64928}