Presentation API: implement WebPresentationClient::setDefaultPresentationUrl().
This is implementing setDefaultPresentationUrl() so the Mojo service is
aware of the new default presentation URL.
The CL also updates some code in the Mojo service implementation in
order to no longer use a default presentation ID.
This is part of a suite of CLs:
1. https://codereview.chromium.org/
1241583004
2. <this>
3. https://codereview.chromium.org/
1234403006
BUG=510484
Review URL: https://codereview.chromium.org/
1245443002
Cr-Commit-Position: refs/heads/master@{#339480}