[Cast Streaming] Fix macOS Desktop Capture with HW Encoders

This patch fixes an issue on macOS where hardware encoders just
present a black screen on macOS due to an invalid assumption about
the VideoFrame format.

In some cases, it looks like the visible_rect of the captured video
frame is cropped for desktop capture. This is quite frankly bizarre.
However the issue is presenting only with H264 because (1) it's the
only HW encoder on macOS and (2) it has to make a copy of the frame
(which ends up not meeting some assumptions around visible_rect, such
as the fact it should be rooted at (0, 0) to use the I420CopyWithPadding
helper function.

This patch also references a new, larger bug about the cropping issue.

Bug: 388316549, 394800925
Change-Id: I01cb2333f3a61139726b1cad17aad2218c2b638a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6239485
Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Jordan Bayles <jophba@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1417073}