Explicit constructor for TreeScope

Fix lint failure.

Change-Id: I89aa665a7d6b375abd8cba0a2768cb6be767d8f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2614818Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841069}

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

@@ -150,7 +150,7 @@ class CORE_EXPORT TreeScope : public GarbageCollectedMixin {
 
  protected:
   TreeScope(ContainerNode&, Document&);
-  TreeScope(Document&);
+  explicit TreeScope(Document&);
   virtual ~TreeScope();
 
   void ResetTreeScope();