Update a couple of focus-related spec steps and their implementations.
The most relevant change is that we no longer allow focusing on elements
that return false for `->is_focusable()`, which necessitates fixing a
broken test that tried to `.focus()` on `<div>`s that were not
focusable. That test's output now more accurately reflects the expected
outcome as seen in other browsers.
The focus chain always consists of newly created GC::Root objects, so
the condition always produced `false`. The fix is to use GC::Root's
overloaded operator== method, which compares the pointers of the stored
type.
This fixes Figma dropdowns and context menus instantly disappearing
upon opening them. This is because they become focused when they insert
them. Because of this bug, it would fire blur events all the way up to
and including the window. Figma listens for the blur event on the
window, and when received, it will instantly hide dropdowns and context
menus. The intention behind this seems to be hiding them when the user
clicks off the browser window, or switches tab.
