Reason for revert:
Breaks Chrome tests.
Original issue's description:
> Reuse allocated encoders in SimulcastEncoderAdapter.
>
> Prior to this change, the SimulcastEncoderAdapter would destroy and create
> encoders whenever it is being reinitialized. After this change, the
> SimulcastEncoderAdapter will cache the already allocated encoders, and reuse
> them after reinitialization.
>
> This change will help in reducing the number of PictureID "jumps" that have
> been seen around encoder reinitialization.
>
> TESTED=AppRTCMobile, Chrome desktop, and internal app, with forced encoder reinits every 30 frames and https://codereview.webrtc.org/2833493003/ applied.
> BUG=webrtc:7475
>
> Review-Url: https://codereview.webrtc.org/2830793005
> Cr-Commit-Position: refs/heads/master@{#18215}
> Committed: 0b8bfb9d98TBR=stefan@webrtc.org,noahric@chromium.org,glaznev@webrtc.org,sprang@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7475
Review-Url: https://codereview.webrtc.org/2893003002
Cr-Commit-Position: refs/heads/master@{#18216}
56e119e2e8