GitLab

Introduced a PresentationServiceClient mojo interface implemented by
PresentationDispatcher. On connection to PresentationService,
PDispatcher will pass a proxy of itself to PSImpl.

Please note that screen availability updates are already throttled
from the browser's perspective.

When a screen availability update is available, it will be transmitted
back to PSDispatcher via the PresentationServiceClient interface.

In the future, I plan to use the same interface for other types of
observer-style APIs (e.g. default session start, session state change,
on message).

Also, simplified the ScreenAvailability API so it only operates on
the DPU (or 1-UA mode if absent). The current semantics to support
listening on arbitrarily presentation URLs + handling behavior for
DPU is very confusing. We should revisit the implementation when we
decide to support it.

This will also hopefully fix the bug where ScreenAvailability mojo
callbacks accumulate over time.

Also fix mojom indent.

BUG=485337

Review URL: https://codereview.chromium.org/1131053005

Cr-Commit-Position: refs/heads/master@{#329457}