Add commandline flag to enable encrypted header extensions for SRTP in WebRTC.

This CL adds a new commandline flag and chrome://flags option to enable
negotiation of encrypted header extensions for SRTP in WebRTC as defined
in RFC 6904.

BUG=webrtc:3411

Change-Id: Iaa2f21a337be79a6dc052020344427183b427bbc
Reviewed-on: https://chromium-review.googlesource.com/563622
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Taylor Brandstetter <deadbeef@chromium.org>
Commit-Queue: Joachim Bauch <jbauch@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#496680}
8 files changed