Simplify SharedWorkerHost

This CL gets rid of the "closed" state of SharedWorkerHost.
This is done by deleting the host instance when it needs to
be terminated.

Adds a TestSharedWorkerServiceImpl that allows tests to keep
track of running remote shared workers.

Change-Id: I4abe3a72f2c208014e0d406f12210da74a3f1971
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758981
Commit-Queue: Patrick Monette <[email protected]>
Reviewed-by: Matt Falkenhagen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#692124}
16 files changed