Reland of Make RtcEventLogImpl to use a TaskQueue instead of a helper-thread ( https://codereview.webrtc.org/3007473002/ )

Original issue's description:
> Revert of Make RtcEventLogImpl to use a TaskQueue instead of a helper-thread (patchset #18 id:340001 of https://codereview.webrtc.org/3007473002/ )
>
> Reason for revert:
> Breaks google3 project.
>
> Original issue's description:
> > Make RtcEventLogImpl to use a TaskQueue instead of a helper-thread
> >
> > Make RtcEventLogImpl to use a TaskQueue instead of a helper-thread. This will eventually allow us to run multiple log sessions on a single task-queue.
> >
> > BUG=webrtc:8142, webrtc:8143, webrtc:8145
> >
> > Review-Url: https://codereview.webrtc.org/3007473002
> > Cr-Commit-Position: refs/heads/master@{#19666}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/f33cee7534f0469330b6b7aa57f84eec4b082598

TBR=terelius@webrtc.org,nisse@webrtc.org,charujain@webrtc.org
NOPRESUBMIT=true
NOTRY=True
BUG=webrtc:8142, webrtc:8143, webrtc:8145

Review-Url: https://codereview.webrtc.org/3012783002
Cr-Original-Commit-Position: refs/heads/master@{#19712}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 248fd4f47c073cc4b444d31c236e3d091487c565