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,eladalon@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:8142, webrtc:8143, webrtc:8145

Review-Url: https://codereview.webrtc.org/3010143002
Cr-Original-Commit-Position: refs/heads/master@{#19672}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3eac8002db8fdf5124026d5377a1f3ec1d16bb0d
5 files changed