Remove lock for process thread pointer from PacedSender.

Also adding code in preparation of hiding the Module
implementation in PacedSender. The implementation details of
how the PacedSender+ProcessThread interaction works, has now
been moved into PacedSender (and out of RtpTransportControllerSend).

Instead of adding a "GetModuleImplementationForTesting" method
to the PacedSender class (which would have been the lazy way
out), I incorporated MockedProcessThread in the PacedSender tests.
This means more boilerplate code but the Module functionality
can be tested separately from the PacedSender and down the line
I think it would be a good idea to start using a separate thread
in the test, which is how the class under test is really used
in production.

Bug: none
Change-Id: Iec1b7c97cb0b363b331143ca70545e6ebafe2cd4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/149176
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29011}