<slot> and active insertion points returns false from canParticipateInFlatTree(), but those elements can have a style, especially in this new display: contents world.

Usage of canParticipateInFlatTree() has created a bunch of bugs:

- Document::styleForElementIgnoringPendingStylesheets does:

  if (!element->canParticipateInFlatTree())
    return ensureStyleResolver().styleForElement(element, nullptr);

This means anyone doing styleForElementIgnoringPendingStylesheets on a <slot> is getting a totally wrong answer for the slot's style, while a div with display: contents does correctly get a style.

- Document::needsLayoutTreeUpdateForNode does:

  if (!node.canParticipateInFlatTree())
    return false;

This means anyone using Document::updateStyleAndLayoutTreeForNode gets the wrong answer for slots.

In fact the only reason some of the code appears to work today is that getComputedStyle() always unconditionally forces a layout when an element is in a shadow tree (which is wrong).

<slot> and all other display: contents things need to be treated the same by the style system and we need to remove all usage of canParticipateInFlatTree() inside it if we want these nodes to have styles.

ex. of a test that only passes because the system forces a recalc today:
fast/alignment/parse-alignment-of-root-elements.html

This styles a <slot> in a shadow tree and uses getComputedStyle() on it, which only works because of the bug where we force a layout.

This also seems wrong because I'm pretty sure <slot> with display: block should generate a box, and the only reason it doesn't is because of display: contents. 

Today the LayoutTreeTraversal skips all slots, but does return display: contents things for parent(), but not for layoutParent(). We inherit the style from parent, but not from layoutParent(), which we only use for checks in StyleAdjuster. This doesn't work though, because <slot>'s are just display: contents things in the UA sheet.

LayoutTreeTraversal::parent() must return insertion points to get the inheritance chain correct.