Reland of use allocated encoders in SimulcastEncoderAdapter. (patchset #1 id:1 of https://codereview.webrtc.org/2893003002/ )

Reason for reland:
Chrome encoder implementation fixed.

Original issue's description:
> Revert of Reuse allocated encoders in SimulcastEncoderAdapter. (patchset #15 id:320001 of https://codereview.webrtc.org/2830793005/ )
>
> 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: 0b8bfb9d98
>
> TBR=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}
> Committed: 56e119e2e8

TBR=stefan@webrtc.org,noahric@chromium.org,glaznev@webrtc.org,sprang@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:7475

Review-Url: https://codereview.webrtc.org/2901493002
Cr-Commit-Position: refs/heads/master@{#18228}

webrtc/modules/video_coding/BUILD.gn

@@ -231,6 +231,7 @@ rtc_static_library("webrtc_vp8") {
     "../..:webrtc_common",
     "../../api/video_codecs:video_codecs_api",
     "../../base:rtc_base_approved",
+    "../../base:rtc_task_queue",
     "../../common_video",
     "../../system_wrappers",
   ]