If gather_continually is set to true, keep the last port allocator session running while stopping all existing process of getting ports (when p2ptransportchannel first becomes writable).

BUG=5034

Review URL: https://codereview.webrtc.org/1359363003

Cr-Original-Commit-Position: refs/heads/master@{#10110}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 98db68fdaa12c5bfca8b0004eba24d034f32de71
7 files changed