Bug 1645528 - Connect nsRefreshDrivers in content processes with a...

Bug 1645528 - Connect nsRefreshDrivers in content processes with a widget-local vsync source r=mattwoodrow,emilio

To allow `requestAnimationFrame()` and similar things to run at monitor
speed if there is only a window-specific vsyncsource available.
This is the case for Wayland and, in the future, EGL/X11. Other backends
may opt for window specific vsyncsources as well at some point.

The idea is to, instead of using global vsync objects, expose a vsyncsource
from nsWindow and use it for refresh drivers. For the content process, move
VsyncChild to BrowserChild, so for each Browserchild there is only one
VsyncChild to which all refresh drivers connect.

IPC in managed either by PBrowser or PBackground. Right now, PBrowser is
only used on Wayland, as both PBrowser and the Wayland vsyncsource run
on the main thread. Other backends keep using the background thread for
now.

While at it, make it so that we constantly update the refresh rate. This
is necessary for Wayland, but also on other platforms variable refresh rates
are increasingly common. When using PVsync, limit updates to once in every
250ms in order to minimize overhead while still updating fast.

How to test:
 - run the Wayland backend
 - enable `widget.wayland_vsync.enabled`
 - optionally: disable `privacy.reduceTimerPrecision`
 - run `vsynctester.com` or `testufo.com`

Expected results:
Instead of fixed 60Hz, things should update at monitor refresh rate -
e.g. 144Hz

Original patch by Kenny Levinsen.

Differential Revision: https://phabricator.services.mozilla.com/D93173