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As Service Worker IPCs are all in Mojo now, we no longer use
ServiceWorkerDispatcherHost to handle any legacy IPCs, its only usage is
to implement mojom::ServiceWorkerDispatcherHost interface, so, it no
longer needs to be a BrowserMessageFilter.
This CL makes ServiceWorkerDispatcherHost not a BrowserMessageFilter any
more, but still keeps its Mojo interface being associated with the
legacy IPC channel.

In future once we make clear of those potential races with some legacy
IPCs like navigation IPCs, we can consider putting
mojom::ServiceWorkerDispatcherHost on a dedicated Mojo message pipe, or
we'd remove mojom::ServiceWorkerDispatcherHost completely if we can find
an alternative/clear way to achieve the same goal.

BUG=845341

Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I9f764017832a5e2f9ff15aff9b49b4c2a6834b78
Reviewed-on: https://chromium-review.googlesource.com/1105623Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Han Leon <leon.han@intel.com>
Cr-Commit-Position: refs/heads/master@{#569232}