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The pieces added are as follows:

1. PresentationServiceDelegate: An interface to be implemented by an
embedder to handle Presentation API requests. Includes an Observer
interface to observe the lifetime of the delegate object.
2. PresentationScreenAvailabilityListener: An interface to be
implemented by content/ code to receive screen availability results
from the embedder.
3. Added method to ContentBrowserClient: GetPresentationServiceDelegate.
Returns nullptr by default. Embedder may override it with an
implementation that can handle Presentation API requests. Added to
allow content/ code to obtain a reference to a
PresentationServiceDelegate to forward presentation API requests to.

Presentation Mojo service patch: crrev.com/839773002/
Blink plumbing: crrev.com/832263007

The Chrome implementation of PresentationServiceDelegate is currently
in a private repo.

BUG=412331

Review URL: https://codereview.chromium.org/883953002

Cr-Commit-Position: refs/heads/master@{#313563}