message: Allow nil degradationPreference in RTCRtpParameters.
  Cr-Commit-Position: refs/heads/master@{#30169}
diff: @@ -48,7 +48,7 @@ RTC_OBJC_EXPORT
  * Degradation preference in case of CPU adaptation or constrained bandwidth.
  * If nil, implementation default degradation preference will be used.
  */
  -@property(nonatomic, copy) NSNumber *degradationPreference;
  +@property(nonatomic, copy, nullable) NSNumber *degradationPreference;
  - (instancetype)init NS_DESIGNATED_INITIALIZER;
  