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This is a reland of dec0aa0b

To make it easier to reland, this CL now only adds the worker nodes to
the PM graph but does not set the connection to its client frame.

Original change's description:
> [PM] Add support for dedicated workers
>
> The WorkerWatcher observes dedicated workers through the
> DedicatedWorkerService interface.
>
> Change-Id: If62d911e500def7ebb905311485a8d89eeaca8df
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992208
> Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
> Commit-Queue: Patrick Monette <pmonette@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#736411}

Change-Id: I7d11799402eb7871e5b8dcbb1b682996eaf12b6f
Bug: 993029
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037498Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739401}