Currently Shadow DOM related files are scattered between dom/ and html/.
It could be organized in some better way:
- ShadowRoot and TreeScope: under dom/ (Doesn't change)
- ComposedTreeWalker: under dom/shadow/ (Move from html/shadow)
- ContentDistributor family and InsertionPoint: under dom/shadow/ (Move from html/shadow)
By doing this, we can
- eliminate dom-to-html dependency by moving distribution related code to dom/shadow/
- Clarify what part is SHADOW_DOM guarded code (which is under dom/shadow)