GitLab

Move StartCastComponent() into the test fixture, and break out the
CastChannel implementation to a separate helper class, for clarity.

Also fixes WebComponent to be usable even if the caller does not
provide a service-directory request handle to CreateComponent().

Bug: 918724, 893229
Change-Id: I9fafd6536d550240d79ee7baa7f4e653db823066
Reviewed-on: https://chromium-review.googlesource.com/c/1474647Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633847}