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the chromoting host.
It only modifies webapp behavior if the "casting" capability is part of the
negotiated set between host and client.

It performs the following tasks:
1. Sends and receives extension messages to/from the host.
2. Initializes and uses the Google Cast Chrome Sender API library to interact
with nearby Cast Receivers, acting as a Sender App.
3. Acts as a message proxy between the Cast Host Extension and the Cast
Receiver, brokering their peer connection.

Review URL: https://codereview.chromium.org/464133003

Cr-Commit-Position: refs/heads/master@{#290140}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290140 0039d316-1c4b-4281-b951-d872f2087c98