In bug 1637500 we made sure that GetCurrentSerialEventTarget returned the properly value with:

• TaskQueue
• STS
• XPCOMThreadWrapper/AbstractThread

We had a pending P5 change that made the DocGroup's serial event target being returned however, as mentioned by :nika we want to keep returning the actual main thread.

We also covered the MessageLoop::EventTarget; however, we want to return the actual XPCOM nsThread if used by the MessageLoop.

In the case where the MessageLoop doesn't have a XPCOM nsThread; we wants to return the MessageLoop's EventTarget.
This will also cover cases where in process with XPCOM not initilised (such as GMP); GetCurrentSerialEventTarget will return a non-null value.

EventTarget::Dispatch can be called on any threads ; there's a potential of a race when accessing the mLoop member.

It can be used to wrap a Runnable and set the CurrentSerialEventTarget TLS for the duration of the runnable's run and so that it returns the right value when called.

We want it to returning the actual nsThread if that's where the MessageLoop would dispatch its tasks; otherwise return the MessageLoop's EventTarget

Depends on D80357

Backed out 2 changesets (bug 1647958) for conflicting with the backout of Bug 1648898.
https://hg.mozilla.org/integration/autoland/rev/a4619f6a9a75434109982924d7e0809358c20adf

Push with failures:
https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=pending%2Crunning%2Csuccess%2Ctestfailed%2Cbusted%2Cexception&revision=0e0f336c4e3b06a6df9d11d00a1ae695d41e64b7&selectedTaskRun=FwtRbAPrTZK7XpRTWq8VGw.0

Failure log:
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=307992480&repo=autoland&lineNumber=1654

https://hg.mozilla.org/mozilla-central/rev/1080acf92cfc
https://hg.mozilla.org/mozilla-central/rev/9a07058c8c33