Deprecate EncodedImageCallback::OnDroppedFrame.

All notifications by a video encoder about a dropped frame should be
done via EncodedImageCallback::OnFrameDropped(uint32_t rtp_timestamp,
int spatial_id, bool is_end_of_temporal_unit) instead. This allows
WebRTC to reason about which frame it is an what the consequences are.

Note that in a future CL, the old method will be removed and the new
one will be made pure virtual. It will be made mandatory that encoders
report the end_of_temporal_unit in either EncodedImage and or via the
OnFrameDropped callback, for each frame.

Bug: webrtc:467444018
Change-Id: Ifb03cc06184a4c7106ffd2d8c33c1f9892b5dd12
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/451940
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47419}