Fixing bug where Connection drops packets when presumed writable.

The "should I simulate EWOULDBLOCK?" determination now happens
solely in P2PTransportChannel. This also fixes a bug where the
"last packet id" was set even if no packet was sent.

R=honghaiz@webrtc.org, pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/2099783002 .

Cr-Original-Commit-Position: refs/heads/master@{#13307}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6bb1ef2b868fef645fdddb41599d6a6693b634d8
17 files changed