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}
diff --git a/api/video_codecs/scalability_mode.h b/api/video_codecs/scalability_mode.h
new file mode 100644
index 0000000..e5809ed
--- /dev/null
+++ b/api/video_codecs/scalability_mode.h
@@ -0,0 +1,40 @@
+/*
+ *  Copyright (c) 2022 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef API_VIDEO_CODECS_SCALABILITY_MODE_H_
+#define API_VIDEO_CODECS_SCALABILITY_MODE_H_
+
+namespace webrtc {
+
+// Supported scalability modes. Most applications should use the
+// PeerConnection-level apis where scalability mode is represented as a string.
+// This list of currently recognized modes is intended for the api boundary
+// between webrtc and injected encoders. Any application usage outside of
+// injected encoders is strongly discouraged.
+enum class ScalabilityMode {
+  kL1T1,
+  kL1T2,
+  kL1T3,
+  kL2T1,
+  kL2T1h,
+  kL2T1_KEY,
+  kL2T2,
+  kL2T2_KEY,
+  kL2T2_KEY_SHIFT,
+  kL2T3_KEY,
+  kL3T1,
+  kL3T3,
+  kL3T3_KEY,
+  kS2T1,
+  kS3T3,
+};
+
+}  // namespace webrtc
+#endif  // API_VIDEO_CODECS_SCALABILITY_MODE_H_