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This CL adds two fields to the default web app config JSON:
 - load_and_await_service_worker_registration: bool
 - service_worker_install_url: URL string

These allow default apps to control their service worker registration
behaviour during the install flow.

Bug: 1123435
Change-Id: Id389c347066a8dbcaa2c2350b884cdb5872e5721
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2409057
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Glen Robertson <glenrob@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807358}