Fork DesktopResizerX11 into DesktopResizerX11 and X11DesktopResizer.

Renames DesktopResizerX11 to X11DesktopResizer and then re-adds
DesktopResizerX11 as a small wrapper which calls X11DesktopResizer.

This is part of a series of CLs to ultimately split the main logic of
DesktopResizerX11 into a different file which uses gfx:: instead of
webrtc structs.

Bug: 40257169
Change-Id: Idc3537f7a3c5562e3c4a751fe45d7cfaf20b5d14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5435609
Commit-Queue: Brad Triebwasser <btriebw@chromium.org>
Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1285354}