(Reland) Service Worker: Handle same-scope, new script registration

Reland of 2da192aff after:
- disabling some layout tests that need to be rebaselined
- landing 6cce6b78f "Decouple script_url from ServiceWorkerRegistration"
so Registration doesn't get in a weird state with a script_url that failed to
register

Before this patch, register() would delete an existing registration at
the scope if the script URL didn't match, and register a new one. This
overwriting creates a scenario where old tabs have a different
controller than new tabs, which the Service Worker spec avoids.

This patch implements the spec steps for same-scope, new script
register(). That means:
- If the existing registration is uninstalling, wait for that to
complete before doing anything.
- Create a new worker which becomes the installing worker of the
existing registration.

BUG=398355
TEST=https://codereview.chromium.org/480943002/

Review URL: https://codereview.chromium.org/506043002

Cr-Commit-Position: refs/heads/master@{#292106}

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