DegradationPreference is already available in namespace webrtc so looks
like there is no reason to redeclare it. Also it cause compilation
error with GCC 5.4.0
Bug: webrtc:10792
Change-Id: I814e90000b8692de67ea477ea7d2769a34a14f01
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144523
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28470}