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)