[Cast Streaming] Fix and cleanup performance overlay

Currently, the performance overlay can cause some CHECK errors due
to inappropriate accesses of base::spans during copying VideoFrame
objects. This patch fixes that bug by moving to use libyuv similar
to how other users of media::VideoFrame copy the frame content, since
media::VideoFrame does not provide any such mechanisms (on purpose,
I'm sure).

This patch also just cleans up this class, doing a little bit better job
of separating data from logic and hopefully being more legible.

Change-Id: I71413135ae90dabf17f1dda5f7c3990333064d23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6192450
Commit-Queue: Jordan Bayles <jophba@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1410096}