IntersectionObserver: use nullptr for implicit root.

third_party/WebKit/Source/core/dom/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//third_party/WebKit/Source/core/core.gni")
 
 blink_core_sources("dom") {
   split_count = 5
 
   sources = [
@@ skipping 106 matching lines @@
     "DocumentTiming.h",
     "DocumentType.cpp",
     "Element.cpp",
     "Element.h",
     "ElementData.cpp",
     "ElementData.h",
     "ElementDataCache.cpp",
     "ElementDataCache.h",
     "ElementFullscreen.cpp",
     "ElementFullscreen.h",
+    "ElementIntersectionObserverData.cpp",
+    "ElementIntersectionObserverData.h",
     "ElementRareData.cpp",
     "ElementRareData.h",
     "ElementTraversal.h",
     "ElementVisibilityObserver.cpp",
     "ElementVisibilityObserver.h",
     "EmptyNodeList.cpp",
     "EmptyNodeList.h",
     "ExecutionContext.cpp",
     "ExecutionContext.h",
     "ExecutionContextTask.h",
@@ skipping 50 matching lines @@
     "NameNodeList.cpp",
     "NameNodeList.h",
     "NamedNodeMap.cpp",
     "Node.cpp",
     "NodeChildRemovalTracker.cpp",
     "NodeChildRemovalTracker.h",
     "NodeComputedStyle.h",
     "NodeFilter.cpp",
     "NodeFilter.h",
     "NodeFilterCondition.h",
-    "NodeIntersectionObserverData.cpp",
-    "NodeIntersectionObserverData.h",
     "NodeIterator.cpp",
     "NodeIterator.h",
     "NodeIteratorBase.cpp",
     "NodeIteratorBase.h",
     "NodeList.cpp",
     "NodeListsNodeData.cpp",
     "NodeListsNodeData.h",
     "NodeRareData.cpp",
     "NodeRareData.h",
     "NodeTraversal.cpp",
@@ skipping 182 matching lines @@
     "shadow/SlotAssignment.h",
     "shadow/SlotScopedTraversal.cpp",
     "shadow/SlotScopedTraversal.h",
   ]
 
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 }