NeedsRebuildLayoutTree after SlotInFlatTree removal.

Runtime flag is now removed so adjusted comment accordingly.
IsActiveV0InsertionPoint required pulling in v0_insertion_point.h, so
dropped doing that.

Change-Id: I914c1567c6a90473b53b470ff34d35b32b2530d2
Reviewed-on: https://chromium-review.googlesource.com/1209502Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589435}

third_party/blink/renderer/core/dom/element.h

@@ -508,9 +508,8 @@ class CORE_EXPORT Element : public ContainerNode {
   void RecalcStyle(StyleRecalcChange);
   bool NeedsRebuildLayoutTree(
       const WhitespaceAttacher& whitespace_attacher) const {
-    // TODO(futhark@chromium.org): !CanParticipateInFlatTree() can be replaced
-    // by IsActiveV0InsertionPoint() when slots are always part of the flat
-    // tree, and removed completely when Shadow DOM V0 support is removed.
+    // TODO(futhark@chromium.org): !CanParticipateInFlatTree() can be removed
+    // when Shadow DOM V0 support is removed.
     return NeedsReattachLayoutTree() || ChildNeedsReattachLayoutTree() ||
            !CanParticipateInFlatTree() ||
            (whitespace_attacher.TraverseIntoDisplayContents() &&