Fix grey bar when viewing local capture in Google Meet

Meet gives an option to request video quality of 360. Not all cameras
will natively return a height-360 size, so Chrome will go with the
next-biggest capture size, and letterbox-and-scale the video frame.

On the machine where I reproduced this, the media::VideoFrame had the
following attributes.
- visible_rect:0,60 640x360
- natural_size:640x360
- coded_size:640x480

This exposed the following bug in the combination of the functions
VideoResourceUpdater::CreateForSoftwarePlanes and
PaintCanvasVideoRenderer::Copy.
* The function VideoResourceUpdater::CreateForSoftwarePlanes creates a
  resource of size |coded_size|, and expects that
  PaintCanvasVideoRenderer::Copy will populate the |visible_rect|
  sub-rectangle of that resource.
* PaintCanvasVideoRenderer::Copy actually copies not to |visible_rect|,
  but rather to rectangle at (0,0) that has the size of |visible_rect|.

The consequence is that we end up viewing the resource through a
misaligned letterbox.

The fix is is to inline PaintCanvasVideoRenderer::Copy (which is only a
few lines long) in VideoResourceUpdater::CreateForSoftwarePlanes, and
specify the correct destination rectangle.

Also add some comments to clarify the function's behavior. Note that
this behavior was switched in crrev.com/545920.

Bug: 1090435
Change-Id: Icef81728defc0cf46b35180dada5d68f24e6b21d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2318313


Auto-Submit: ccameron <ccameron@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791480}