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/media/engine/fake_webrtc_video_engine.h b/media/engine/fake_webrtc_video_engine.h
index 0bbddd2..87d107a 100644
--- a/media/engine/fake_webrtc_video_engine.h
+++ b/media/engine/fake_webrtc_video_engine.h
@@ -45,7 +45,7 @@
   ~FakeWebRtcVideoDecoder();
 
   bool Configure(const Settings& settings) override;
-  int32_t Decode(const webrtc::EncodedImage&, bool, int64_t) override;
+  int32_t Decode(const webrtc::EncodedImage&, int64_t) override;
   int32_t RegisterDecodeCompleteCallback(
       webrtc::DecodedImageCallback*) override;
   int32_t Release() override;