This covers the following:
Bug 1413834 Implement sequential focus navigation regarding shadow DOM
> Bug 1430020 Let sequential focus navigation in shadow DOM enter iframes
> Bug 1430701 Handle focus navigation on frameless shadow hosts in light DOM
> Bug 1430692 Handle focus navigation on NAC in shadow DOM
> Bug 1453693 Ensure sequential focus navigation works in Shadow DOM and add some tests
> Bug 1466581 Handle sequential focus also in nested shadow DOM
> Bug 1481079 Shadow DOM hosts should be focusable
> Bug 1507101 Use StyleChildrenIterator instead of custom frame tree walking code to handle NAC inside shadow dom
> Bug 1512043 Ensure traverse all nodes owned by the top level shadow host
> Bug 1512457 Fix various cases that focus navigation doesn't work well with frameless shadow host
> Bug 1513141 Really minor nsFocusManager cleanup
> Bug 1519090 Keyboard focus is trapped inside <slot>
> Bug 1528034 Make IsHostOrSlot null-safe
> Bug 1544826 Wrong focus navigation behavior when the root element is a shadow root
>> Bug 1500273 Ensure backward focus navigation works in Shadow DOM
With the big amount of code churn around DOM a lot of tests
broke severely enough that they caused build bustage.
This commit cleans up, removes or otherwise fixes tests
that are broken, no longer relevant or obsolete.
