ElementVisibilityObserver: use ExecutionContext's document.

third_party/WebKit/Source/core/dom/ElementVisibilityObserver.cpp

 // 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.
 
 #include "core/dom/ElementVisibilityObserver.h"
 
 #include "core/dom/Element.h"
 #include "core/dom/IntersectionObserverEntry.h"
 #include "core/frame/LocalFrame.h"
 #include "wtf/Functional.h"
 
 namespace blink {
 
 namespace {
 
-bool isInRemoteFrame(Element* element) {
-  Frame* mainFrame = element->document().frame()->tree().top();
+bool isInRemoteFrame(const Document& document) {
+  DCHECK(document.frame());
+  Frame* mainFrame = document.frame()->tree().top();
   return !mainFrame || mainFrame->isRemoteFrame();
 }
 
 }  // anonymous namespace
 
 ElementVisibilityObserver::ElementVisibilityObserver(
     Element* element,
     std::unique_ptr<VisibilityCallback> callback)
     : m_element(element), m_callback(std::move(callback)) {}
 
 ElementVisibilityObserver::~ElementVisibilityObserver() = default;
 
 void ElementVisibilityObserver::start() {
+  ExecutionContext* context = m_element->getExecutionContext();
+  DCHECK(context->isDocument());
+  Document& document = toDocument(*context);
+
   // TODO(zqzhang): IntersectionObserver does not work for RemoteFrame.
   // Remove this early return when it's fixed. See https://crbug.com/615156
-  if (isInRemoteFrame(m_element)) {
+  if (isInRemoteFrame(document)) {
     m_element.release();
     return;
   }
 
   DCHECK(!m_intersectionObserver);
   m_intersectionObserver = IntersectionObserver::create(
       Vector<Length>(), Vector<float>({std::numeric_limits<float>::min()}),
-      &m_element->document(),
-      WTF::bind(&ElementVisibilityObserver::onVisibilityChanged,
-                wrapWeakPersistent(this)));
+      &document, WTF::bind(&ElementVisibilityObserver::onVisibilityChanged,
+                           wrapWeakPersistent(this)));
   DCHECK(m_intersectionObserver);
   m_intersectionObserver->observe(m_element.release());
 }
 
 void ElementVisibilityObserver::stop() {
   // TODO(zqzhang): IntersectionObserver does not work for RemoteFrame,
   // so |m_intersectionObserver| may be null at this point after start().
   // Replace this early return with DCHECK when this has been fixed. See
   // https://crbug.com/615156
   if (!m_intersectionObserver)
     return;
 
   m_intersectionObserver->disconnect();
   m_intersectionObserver = nullptr;
 }
 
 DEFINE_TRACE(ElementVisibilityObserver) {
   visitor->trace(m_element);
   visitor->trace(m_intersectionObserver);
 }
 
 void ElementVisibilityObserver::onVisibilityChanged(
     const HeapVector<Member<IntersectionObserverEntry>>& entries) {
   bool isVisible = entries.last()->intersectionRatio() > 0.f;
   (*m_callback.get())(isVisible);
 }
 
 }  // namespace blink