Check for dropped frames by instead checking the
frame_buffer pointer directly.
Also add RTC_DCHECK to verify that a webrtc::VideoFrame never
has video_frame_buffer_ set to nullptr (except by the default
constructor).
BUG=webrtc:5682
Review-Url: https://codereview.webrtc.org/1995343002
Cr-Commit-Position: refs/heads/master@{#12859}