Define RtpGenericFrameDescriptorExtension00

We are about to split RtpGenericFrameDescriptorExtension
into v00 and v01. Allow downstream projects to refer to
RtpGenericFrameDescriptorExtension00 now, so that we may later
delete references to RtpGenericFrameDescriptorExtension
without breaking their build.

Bug: webrtc:10214
Change-Id: I45528699bf7d8cc6c22c22a601f248cca2ba6c90
Reviewed-on: https://webrtc-review.googlesource.com/c/121769
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26588}
diff --git a/modules/rtp_rtcp/source/rtp_generic_frame_descriptor_extension.h b/modules/rtp_rtcp/source/rtp_generic_frame_descriptor_extension.h
index 0d673e0..5c1bc52 100644
--- a/modules/rtp_rtcp/source/rtp_generic_frame_descriptor_extension.h
+++ b/modules/rtp_rtcp/source/rtp_generic_frame_descriptor_extension.h
@@ -19,6 +19,10 @@
 
 namespace webrtc {
 
+// TODO(bugs.webrtc.org/10243): Remove when downstream projects stop using
+// RtpGenericFrameDescriptorExtension.
+using RtpGenericFrameDescriptorExtension00 = RtpGenericFrameDescriptorExtension;
+
 class RtpGenericFrameDescriptorExtension {
  public:
   using value_type = RtpGenericFrameDescriptor;