[Fuchsia] Disable exit-when-unused behaviour of Runners.

Exiting the Runner when it is not hosting any components saves memory
but increases the time taken to launch new components, since the
web.Context must be re-created from scratch each time.

Fuchsia's Components v2 APIs will allow the system to teardown Runners
to save resources when required, but for now just run until the system
kills the Runner process/job.

Bug: 952560
Change-Id: I5fdd220fbb1e0587d584f14a5447948e1d1d99bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566766
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652703}
7 files changed