[crd host] Fix garbled frame stats for extrapolated frames

The frame stats is created by WebrtcVideoStream as
WebrtcVideoStream::FrameStats. However, when WebrtcVideoEncoderWrapper
creates a copy of it for extrapolated frames, it uses the copy ctor of
WebrtcVideoEncoder::FrameStats, which is missing some fields. When
WebrtcVideoStream gets back the frame stats, it down-casts it to
WebrtcVideoStream::FrameStats, which causes out-of-bound reads.

This CL fixes the bug by introducing a Duplicate() virtual method that
allows the frame stats to be duplicated with the correct type.

Bug: 437300468
Change-Id: I2cc9d19b4337923c41ae12b46b93b55cd804c6f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6832184
Auto-Submit: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1499232}