Previously, 'now' was set to the time `requestAnimationFrame()` was
called, and the EventLoop's 'now' was ignored. This was a little odd and
meant the time was always in the past.
Instead of using Core::EventLoop and Core::Timer directly, LibWeb now
goes through a Web::Platform abstraction layer instead.
This will allow us to plug in Qt's event loop (and QTimer) over in
Ladybird, to avoid having to deal with multiple event loops.
Previously requestAnimationFrame() callbacks were registered with a
static global RequestAnimationFrameDriver shared between all windows.
This led to callbacks still running after navigating away from a page
(This could be seen with the WASM GoL demo).
This commit moves the RequestAnimationFrameDriver (now
AnimationFrameCallbackDriver) to be a member of the HTML::Window
object, then uses the 'active document' parameter of
run_animation_frame_callbacks() to run only the active callbacks.
