core/page/FocusController.cpp and core/page/SlotScopedNavigation.cpp
use naive algorithm to search next/previous focusable element,
especially when the search is scoped in <slot> scope.

At this point I'm not sure if it was O(N^2), but at least from
what I see now, many O(depth) operations are done in O(N) loop
where N is the number of elements under the slot scope,
thus it can be O(N * depth) at least.

Here's rough analysis (not exhaustive and very accurate, I may miss
something that is "N"):

- ScopedFocusNavigation::NextFocusableElement()
  calls ScopedFocusNavigation::MoveToNext() O(N) times

- ScopedFocusNavigation::MoveToNext()
  calls non-trivial functions:
  - ScopedFocusNavigation::IsSlotFallbackScopedForThisSlot()
  - ScopedFocusNavigation::IsSlotFallbackScoped()
  - SlotScopedTraversal::Next()

ScopedFocusNavigation::IsSlotFallbackScoped(element)
  - O(depth of element)

ScopedFocusNavigation::IsSlotFallbackScopedForThisSlot(slot, current)
  - Loops O(depth of current)
  - calls SlotScopedTraversal::IsSlotScoped() which can take
    O(depth of current)

SlotScopedTraversal::Next()
  - calls NearestInclusiveAncestorAssignedToSlot() which can take
    O(depth)
  - calls NextSkippingChildrenOfShadowHost() O(depth) at worst?
  - loops thorough assigned nodes of a slot, esp. Find() against
    it takes O(number of assigned nodes), then next sibling could
    be found in almost constant time?

Some ideas about more cleanup on the way:
- split ScopedFocusNavigation class out of FocusController.cpp
- define MoveToNext/MovToPrevious/MoveToFirst/MoveToLast as an
  virtual interface and implement in inherited classes, rather than
  using many if-else's in these methods in ScopedFocusNavigation class.