Add flag enabling more packets to be retransmittable.

If not indicated otherwise, allow adding a packet to the retransmission
history at least every 1/15s in order to reduce frame dropping.

BUG=webrtc:7694

Review-Url: https://codereview.webrtc.org/2999063002
Cr-Original-Commit-Position: refs/heads/master@{#19665}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a8ae6f2acaef86e63f349442dbde8e81a9bd041e