desktop_capture: Fix SetFrameDataToBlack() to correctly support YUV/I420 black

Currently, SetFrameDataToBlack() assumes the frame format is always
ARGB/BGRA (4 bytes per pixel packed) and uses a flat memset to 0x00,
which correctly represents transparent black for RGB.

However, for YUV/I420 formats, zero-initializing the buffer results
in Y=0, U=0, V=0, which maps to a distinct medium-dark green in RGB.
When I420 desktop frames are cleared (e.g. inside Chromium's scaling and
pillarbox/letterbox margin rendering pipeline), the margins render
as bright green sidebars instead of black.

This refactors SetFrameDataToBlack() to inspect the pixel_format()
and correctly fill the Y plane to 0x00, and U/V planes to 0x80 (128)
for standard YUV formats (FOURCC_I420, FOURCC_YV12, FOURCC_YU12,
FOURCC_IYUV), yielding grayscale black.

Also adds a new test to desktop_frame_unittest to verify that YUV
black is set correctly.

Bug: webrtc:518064225
Change-Id: Iec72c02084e60dea938b25cf8c2f8301e481526d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/476903
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Jordan Bayles <jophba@google.com>
Reviewed-by: Mark Foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#48140}