Reason for revert:
Seems to be causing new crashes, possibly because of changes to the "Destroy(false)" behavior. Will re-land after investigating these crashes more and addressing the root cause.
Original issue's description:
> Delete SignalThread class.
>
> Rewrite AsyncResolver to use PlatformThread directly, not
> SignalThread, and update includes of peerconnection client to not
> depend on signalthread.h.
>
> BUG=webrtc:6424,webrtc:7723
>
> Review-Url: https://codereview.webrtc.org/2915253002
> Cr-Commit-Position: refs/heads/master@{#18833}
> Committed: bc8feda1dbTBR=tommi@webrtc.org,kwiberg@webrtc.org,nisse@webrtc.org
NOPRESUBMIT=true
NOTRY=true
BUG=webrtc:6424,webrtc:7723
Review-Url: https://codereview.webrtc.org/2979733002
Cr-Commit-Position: refs/heads/master@{#18980}