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StartupContext creates helper classes wrapping the incoming and outgoing
service directory handles, to simplify use of them by component
implementations.

WebRunner & CastRunner are updated to use the StartupInfo helper, and
the CastRunner test StartCastComponent helper is updated to place the
CastChannel service in the component's incoming service directory, for
now.

Bug: 920920
Test: Existing component tests (e.g. cast_runner_integration_tests)
Change-Id: I92e9f0f0aad26aecef0da036684bb66733a267ac
Reviewed-on: https://chromium-review.googlesource.com/c/1473820
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632744}