[remoting] Apply text-scaling-factor for multiple monitors.

Before this CL, DesktopResizerX11 refused to set the
text-scaling-factor if multiple monitors were detected. But the logic
was flawed, because it counted the RANDR "connected" monitors, rather
than just the enabled monitors.

This CL addresses this by setting the text-scaling-factor even if
there are multiple monitors. This is possible because GNOME's X11
backend enforces each monitor to have the same scale - so the
calculation would give correct results for all monitors.

Bug: b:326598074
Change-Id: I407b8bf11917fbebb6ba2592ef3dcac8410b896e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5321097
Auto-Submit: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1264926}