视频质量降级枚举

enum class DegradationPreference {
  // Don't take any actions based on over-utilization signals. Not part of the
  // web API.
  DISABLED,
  // On over-use, request lower frame rate, possibly causing frame drops.
  MAINTAIN_FRAMERATE,
  // On over-use, request lower resolution, possibly causing down-scaling.
  MAINTAIN_RESOLUTION,
  // Try to strike a "pleasing" balance between frame rate or resolution.
  BALANCED,
};