[Adaptation] Add more ResourceAdaptationProcessor logging.

This should help debugging when adaptation is or is not happening
unexpectedly. Log spam is prevented by not logging if the same
result happened to the same resource already and we haven't
adapted since then.

Bug: webrtc:11616
Change-Id: Ia6c5cc35061d252f1c66f2f2bf3b94d2485498d6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/176221
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Evan Shrubsole <eshr@google.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31378}

api/rtp_parameters.h

@@ -92,6 +92,9 @@ enum class DegradationPreference {
   BALANCED,
 };
 
+RTC_EXPORT const char* DegradationPreferenceToString(
+    DegradationPreference degradation_preference);
+
 RTC_EXPORT extern const double kDefaultBitratePriority;
 
 struct RTC_EXPORT RtcpFeedback {