Prepare for injectable SW decoders

Pretty much mirrors the work done on the encoding side in CLs:

"Clean up ownership of webrtc::VideoEncoder"
https://codereview.webrtc.org/3007643002/

"Let VideoEncoderSoftwareFallbackWrapper own the wrapped encoder"
https://codereview.webrtc.org/3007683002/

"WebRtcVideoEngine: Encapsulate logic for unifying internal and external video codecs"
https://codereview.webrtc.org/3006713002/

BUG=webrtc:7925

Review-Url: https://codereview.webrtc.org/3009973002
Cr-Original-Commit-Position: refs/heads/master@{#19641}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 084c55a63a2d9bdc71579458406d44f8bab9f454