nit: Better usage of RGBA/BGRA comments and variable names

Add better comments about ARGB / BGRA / RGBA for FrameSinkVideoCapturer.
ARGB is used by frame_sink_video_capture_impl.cc, the actual buffer
could be BGRA (in CPU, GPU X11 Linux only), or RGBA (in GPU all other
platforms) currently. So, in scope of frame_sink_video_capture_impl.cc,
all RGBA should change to ARGB to let the name keep sync with
user's input format set from SetFormat function (media:: format)

Change-Id: Id90d120a2d0dc24a020c8096246fcb95309ea43b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5418235


Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Reito OvO <carolwolfking@gmail.com>
Reviewed-by: Kyle Charbonneau <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283581}