Reland of Removing references to webrtc::PortAllocatorFactoryInterface.

Relanding after fixing conflict with:
https://codereview.chromium.org/1506383004

Original description:

Now using a new CreatePeerConnection method that takes a
PortAllocator as input. This removes the need for some
boilerplate code and code duplication between webrtc and chromium.
It also fixes an issue with TURN candidate priorities.

BUG=webrtc:5209

Review URL: https://codereview.chromium.org/1516163002

Cr-Commit-Position: refs/heads/master@{#365085}
14 files changed