[Cast] Software encoder support for varying video frame sizes.

Adds support for automatic on-line reconfiguration of the VP8 software
encoder whenever the video frame size changes.  libvpx supports
shrinking frame sizes without tearing down the encoder, but growing
frames sizes still requires a full tear-down and re-creation.

Most of this change involves adding extensive unit testing to confirm
media/cast is now capable of handling variable frames sizes end-to-end.
In addition, the cast_sender_app and cast_receiver_app diagnostic tools
have been updated.

An upcoming change will eliminate the width and height properties from
media::cast::VideoSenderConfig, which are both being ignored now.

BUG=451277

Review URL: https://codereview.chromium.org/877393003

Cr-Commit-Position: refs/heads/master@{#314196}

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