Rename end_of_superframe to end_of_picture.

For consistency with the VP9 RTP spec which uses term "picture" for set
of frames which belong to the same time instance.

Bug: none
Change-Id: I30e92d5debb008feb58f770b63fe10c2e0029267
Reviewed-on: https://webrtc-review.googlesource.com/72180
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23040}
diff --git a/modules/video_coding/include/video_codec_interface.h b/modules/video_coding/include/video_codec_interface.h
index aad3759..2c94093 100644
--- a/modules/video_coding/include/video_codec_interface.h
+++ b/modules/video_coding/include/video_codec_interface.h
@@ -70,7 +70,7 @@
   uint8_t num_ref_pics;
   uint8_t p_diff[kMaxVp9RefPics];
 
-  bool end_of_superframe;
+  bool end_of_picture;
 };
 
 struct CodecSpecificInfoGeneric {