Add L2T3 K-SVC structure

Bug: webrtc:11999
Change-Id: I1bfb8674b95be8155035117c771b5e4c4bfc29c5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208260
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Emil Lundmark <lndmrk@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33299}

modules/video_coding/svc/create_scalability_structure.cc

@@ -54,6 +54,7 @@ constexpr NamedStructureFactory kFactories[] = {
     {"L2T2", Create<ScalabilityStructureL2T2>},
     {"L2T2_KEY", Create<ScalabilityStructureL2T2Key>},
     {"L2T2_KEY_SHIFT", Create<ScalabilityStructureL2T2KeyShift>},
+    {"L2T3_KEY", Create<ScalabilityStructureL2T3Key>},
     {"L3T1", Create<ScalabilityStructureL3T1>},
     {"L3T3", Create<ScalabilityStructureL3T3>},
     {"L3T3_KEY", Create<ScalabilityStructureL3T3Key>},