Before the completion_steps for timer were casted from JS::SafeFunction
to Function in HTML::Timer constructor, which is incorrect because then
callback's captured GC-allocated objects are not protected from being
deallocated. Let's modify HTML::Timer to use JS::HeapFunction for the
callback instead.
By using Core::Timer that accepts Function instead of JS::SafeFunction
in Platform::Timer does we fix memory leak caused by circular
dependency of timer's callback and timer itself.
This change implements a step from the document's destroy procedure in
the specification, saying that all active timers should be cleared.
By doing this, we also fix the leaking of a document in case where we
have navigated away from a page that has scheduled timers that haven't
yet been triggered.
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.
