Remove outdated TODO comment

The comment is no longer true, and we can remove TODO here.

Change-Id: I0b2e4735a543d915c6bc45fcc51fbce6207e4052
Reviewed-on: https://chromium-review.googlesource.com/1248341
Commit-Queue: Hayato Ito <hayato@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594624}

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

@@ -327,9 +327,6 @@ class CORE_EXPORT Node : public EventTarget {
 
   // If this node is in a shadow tree, returns its shadow host. Otherwise,
   // returns nullptr.
-  // TODO(kochi): crbug.com/507413 ownerShadowHost() can return nullptr even
-  // when it is in a shadow tree but its root is detached from its host. This
-  // can happen when handling queued events (e.g. during execCommand()).
   Element* OwnerShadowHost() const;
   // crbug.com/569532: containingShadowRoot() can return nullptr even if
   // isInShadowTree() returns true.