Emit SignalReadyToSend even for "presumed writable" connections.

The Connection class will now blindly forward SignalReadyToSend, and
P2PTransportChannel will decide whether to forward it further (which
it was already doing).

BUG=webrtc:6448

Review-Url: https://codereview.webrtc.org/2374183005
Cr-Original-Commit-Position: refs/heads/master@{#14462}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: dd7fb43f28410e00039660e31e130568c901d03e
2 files changed