[Camera] Update comments around mapped frames in VideoCaptureFeedback.

The feature that added mapped_sizes (InCapturerScaling) was not shipped,
so this field can be deprecated in the future.

Update comment for require_mapped_frame as it is still in active use.

Side note: Frame mapping in the MF capturer requires unsafe shared
memory, which there is an effort to remove entirely; when that effort is
picked up we should see if it can be converted to read-only shmem.

Bug: 1191986,1440076,1353325
Change-Id: Ibd891347a55f2920b73031fba8f482dcfce65edd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4484385
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Mark Foltz <mfoltz@chromium.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1137244}