[WIP] Proxy RtcVideoDecoder calls to a media::VideoDecoder.

[WIP] Proxy RtcVideoDecoder calls to a media::VideoDecoder.

RtcVideoDecoder talks directly to a media::VideoDecodeAccelerator,
managing all of the state necessary to do hardware decoding with a
remote VDA. Instead, proxy calls to a media::GpuVideoDecoder, which
manages this state properly, making RtcVideoDecoder a much-simpler shim
class. This allows us to remove a significant amount of brittle,
dupliucated code, and prepares for the deprecation of VDA in favor of
media::MojoVideoDecoder.

BUG=

Change-Id: I9c36347b44dd7e7f62bd2b6a3380c98892827173

[slan, 2016-10-06 00:58:18]

PS2 works by using an FFMPEG VideoDecoder in the render procress, by enforcing
that VP8 is used:

Sender:   https://apprtc.appspot.com/r/foo?vsc=VP8
Receiver: https://apprtc.appspot.com/r/foo?vrc=VP8

The next step is to host a Mojo Media Service in a remote process (browser),
which is backed by FFMPEG Decoder.