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-Original-Commit-Position: refs/heads/master@{#19666}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f33cee7534f0469330b6b7aa57f84eec4b082598
5 files changed