rtc::Thread::PostTask() added.

This method allows asynchronously posting tasks, in the form of
functors to be invoked, on the thread represented by rtc::Thread.

This CL removes PostMessageWithFunctor(), putting the implementation of
it directly into rtc::Thread::PostTask(), and moves & updates the test
coverage to thread_unittest.cc.

Bug: webrtc:10294, webrtc:10293
Change-Id: Ic6cc3e2533a4f7aaff141aff28e9bb3908ee3c96
Reviewed-on: https://webrtc-review.googlesource.com/c/124701
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26894}