New enum ScalabilityMode.

Used instead of string representation in lower-levels of encoder configuration, to avoid string comparisons (with risk of misspelling) in lots of places.

Bug: webrtc:11607
Change-Id: I4d51c2265aac297c29976d2aa601d8ffb33b7326
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/259870
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36706}

api/video_codecs/video_encoder_config.h

@@ -18,6 +18,7 @@
 
 #include "absl/types/optional.h"
 #include "api/scoped_refptr.h"
+#include "api/video_codecs/scalability_mode.h"
 #include "api/video_codecs/sdp_video_format.h"
 #include "api/video_codecs/video_codec.h"
 #include "rtc_base/ref_count.h"
@@ -64,7 +65,7 @@ struct VideoStream {
   // between multiple streams.
   absl::optional<double> bitrate_priority;
 
-  absl::optional<std::string> scalability_mode;
+  absl::optional<ScalabilityMode> scalability_mode;
 
   // If this stream is enabled by the user, or not.
   bool active;