Remove EncodedFrame::MissingFrame and start removing Decode() param

Remove EncodedFrame::MissingFrame, as it was always false in actual
in-use code anyway, and remove usages of the Decode missing_frames param
within WebRTC. Uses/overrides in other projects will be cleaned up
shortly, allowing that variant to be removed from the interface.

Bug: webrtc:15444
Change-Id: Id299d82e441a351deff81c0f2812707a985d23d8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/317802
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Tony Herre <herre@google.com>
Commit-Queue: Tony Herre <herre@google.com>
Cr-Commit-Position: refs/heads/main@{#40662}
diff --git a/modules/video_coding/generic_decoder.h b/modules/video_coding/generic_decoder.h
index be08fc6..b1fb1f3 100644
--- a/modules/video_coding/generic_decoder.h
+++ b/modules/video_coding/generic_decoder.h
@@ -120,8 +120,7 @@
  private:
   int32_t Decode(const EncodedImage& frame,
                  Timestamp now,
-                 int64_t render_time_ms,
-                 int64_t missing_frame);
+                 int64_t render_time_ms);
   VCMDecodedFrameCallback* _callback = nullptr;
   VideoDecoder* const decoder_;
   VideoContentType _last_keyframe_content_type;