Adding support for HDR headroom in ScreenDetailed

Hello Second Screen WG (and ColorWeb-CG as FYI)!

The HDR headroom of a screen is a core parameter for how HDR content is
rendered (see this description in CSS HDR
<https://drafts.csswg.org/css-color-hdr/#introducing-headroom>). Advanced
applications may want to perform custom tone mapping using this parameter.
This is another brick in the HDR canvas proposal
<https://github.com/w3c/ColorWeb-CG/blob/hdr_canvas_r2/hdr_html_canvas_element.md>
that
has been slowly making progress.

The short version of the proposal is: Add the screen's HDR headroom
parameter to ScreenDetailed. It is a potential fingerprinting vector, so
ScreenDetailed is a better fit than Screen. It can also fluctuate rapidly
(e.g, when the screen brightness changes) and independently of other screen
properties, and so it gets its own "on change" event, separate from the
other screen properties.

Please see below for the fully fleshed out proposal:

   - Issue <https://github.com/w3c/window-management/issues> on
   window-management API
   - PR <https://github.com/w3c/window-management/pulls> on
   window-management API
   - Explainer
   <https://github.com/ccameron-chromium/ColorWeb-CG/blob/screen_hdr_headroom/screen_hdr_headroom.md>

This has been shipping behind a flag for several years (although with some
slight API differences, which have been ironed out to better fit the CSS
HDR model). Now that HDR content is getting to be much more common, there
have been several requests to get this out from behind the flag.