Reland of Prepare for injectable SW decoders (patchset #1 id:1 of https://codereview.webrtc.org/3010953002/ )

Reason for revert:
Fix bug introduced by keeping the allocated decoders in a map.

Original issue's description:
> Revert of Prepare for injectable SW decoders (patchset #3 id:40001 of https://codereview.webrtc.org/3009973002/ )
>
> Reason for revert:
> Tentative revert since it seems to cause problems in Chrome, MAC.
>
> https://build.chromium.org/p/chromium.webrtc.fyi/builders/Mac%20Tester/builds/42684
>
>
>
> Original issue's description:
> > 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-Commit-Position: refs/heads/master@{#19641}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/084c55a63a2d9bdc71579458406d44f8bab9f454
>
> TBR=magjed@webrtc.org,andersc@webrtc.org
> # Not skipping CQ checks because original CL landed more than 1 days ago.
> BUG=webrtc:7925
>
> Review-Url: https://codereview.webrtc.org/3010953002
> Cr-Commit-Position: refs/heads/master@{#19647}
> Committed: https://chromium.googlesource.com/external/webrtc/+/1f88531038c24c5ce3b0f4cfc682b970770a71f6

TBR=magjed@webrtc.org,perkj@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:7925

Review-Url: https://codereview.webrtc.org/3005363002
Cr-Original-Commit-Position: refs/heads/master@{#19782}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 063f0c0d3ac62f9110d54c7fea8aed7722264600
10 files changed