Add ServiceWorkerWorkerClientRegistry mojo interface

So that later it can be used to register a new ServiceWorkerWorkerClient
from non-main-thread for nested workers.

No behavior changes (no nested worker plumbing added here)

Bug: 31666
Change-Id: I0a9b52b47a5eb5eb234da5c18a6bf15c65e14d2c
Reviewed-on: https://chromium-review.googlesource.com/1107580
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569461}
7 files changed