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This patch introduces a WiFi Display session class
skeleton and mojo service which provides network
access for the render-hosted session.

The introduced code is compiled only if a newly added
'enable_wifi_display' build option is set.

Besides, some minor changes were applied to the
DisplaySourceConnectionDelegate interface in order
to better define its methods behavior.

BUG=242107

Review URL: https://codereview.chromium.org/1540563002

Cr-Commit-Position: refs/heads/master@{#369415}