Reland "[fuchsia] Launch web_instance.cm from context_provider.cm"

This reverts commit e9df2842.

Reason for reland: The various routing issues for context_provider.cm
have been resolved. Also fixed an issue with coverage builds, which
were enabled after the revert.

Original change's description:
> Revert "[fuchsia] Launch web_instance.cm from context_provider.cm"
>
> This reverts commit 5e3215ba.
>
> Reason for revert: Appears to fail upstream E2E tests, due to various
> capabilities not being available to the web_instances.
>
> Original change's description:
> > [fuchsia] Launch web_instance.cm from context_provider.cm
> >
> > In so doing:
> >
> > - Add support for CreateContextParams.service_directory in
> >   WebInstanceHost.
> > - Add support to WebInstanceHost for running within a process with
> >   `--with-webui` on its command line to launch child instances in the
> >   `web_engine_with_webui` package rather than the `web_engine`
> >   package. This feature is used by `web_engine_shell` in test scenarios.
> >
> > Bug: 1280703
> > Fuchsia-Binary-Size: Size increase is unavoidable.
> > Change-Id: I08d65492c4c670e67cca88beb223d1d8c8eed76f
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4194689


> > Reviewed-by: Rohan Pavone <rohpavone@chromium.org>
> > Reviewed-by: Wez <wez@chromium.org>
> > Commit-Queue: Greg Thompson <grt@chromium.org>
> > Cr-Commit-Position: refs/heads/main@{#1108989}
>
> Bug: 1280703
> Bug: b/270955577
> Change-Id: Ifc61fc5ec781f5624551310f7a0b968c2d1ad348
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4294973


> Reviewed-by: Rohan Pavone <rohpavone@chromium.org>
> Auto-Submit: Wez <wez@chromium.org>
> Reviewed-by: Alex Gough <ajgo@chromium.org>
> Commit-Queue: Wez <wez@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1111658}

Bug: 1280703, 1421351, 1426185
Bug: b/270955577
Fuchsia-Binary-Size: Size increase is unavoidable.
Change-Id: Ic1cc04f8420853f31c549fa9fe99b79b24b419dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4313066


Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: David Dorwin <ddorwin@chromium.org>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Auto-Submit: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1120051}