Monitor the intersection of video and viewport.

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

Index: third_party/WebKit/Source/core/dom/ElementVisibilityObserver.cpp
diff --git a/third_party/WebKit/Source/core/dom/ElementVisibilityObserver.cpp b/third_party/WebKit/Source/core/dom/ElementVisibilityObserver.cpp
index 8c4ecab9c24d74549216268dbf9a3f38e3310e9a..0a5fbc3246d06a3e522a1aff72ecbb4675ddddaa 100644
--- a/third_party/WebKit/Source/core/dom/ElementVisibilityObserver.cpp
+++ b/third_party/WebKit/Source/core/dom/ElementVisibilityObserver.cpp
@@ -29,6 +29,10 @@ ElementVisibilityObserver::ElementVisibilityObserver(
 ElementVisibilityObserver::~ElementVisibilityObserver() = default;
 
 void ElementVisibilityObserver::start() {
+  start(Vector<float>({std::numeric_limits<float>::min()}));
+}
+
+void ElementVisibilityObserver::start(const Vector<float>& thresholds) {
   ExecutionContext* context = m_element->getExecutionContext();
   DCHECK(context->isDocument());
   Document& document = toDocument(*context);
@@ -42,9 +46,9 @@ void ElementVisibilityObserver::start() {
 
   DCHECK(!m_intersectionObserver);
   m_intersectionObserver = IntersectionObserver::create(
-      Vector<Length>(), Vector<float>({std::numeric_limits<float>::min()}),
-      &document, WTF::bind(&ElementVisibilityObserver::onVisibilityChanged,
-                           wrapWeakPersistent(this)));
+      Vector<Length>(), thresholds, &document,
+      WTF::bind(&ElementVisibilityObserver::onVisibilityChanged,
+                wrapWeakPersistent(this)));
   DCHECK(m_intersectionObserver);
   m_intersectionObserver->setInitialState(
       IntersectionObserver::InitialState::kAuto);
@@ -74,7 +78,8 @@ DEFINE_TRACE(ElementVisibilityObserver) {
 
 void ElementVisibilityObserver::onVisibilityChanged(
     const HeapVector<Member<IntersectionObserverEntry>>& entries) {
-  bool isVisible = entries.last()->intersectionRatio() > 0.f;
+  m_ratio = entries.last()->intersectionRatio();
+  bool isVisible = m_ratio > 0.f;
   (*m_callback.get())(isVisible);
 }