Use intersection observer to control frame throttling

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 06769a487de85306bb853729c2cf2f6163d92347..f04274ba179159d2cb59c98e03ac65600c3a244a 100644
--- a/third_party/WebKit/Source/core/dom/ElementVisibilityObserver.cpp
+++ b/third_party/WebKit/Source/core/dom/ElementVisibilityObserver.cpp
@@ -47,6 +47,8 @@ void ElementVisibilityObserver::start() {
                            wrapWeakPersistent(this)));
   DCHECK(m_intersectionObserver);
   m_intersectionObserver->observe(m_element.release());
+  m_intersectionObserver->setInitialState(
+      IntersectionObserver::InitialState::AUTO);
 }
 
 void ElementVisibilityObserver::stop() {
@@ -61,6 +63,10 @@ void ElementVisibilityObserver::stop() {
   m_intersectionObserver = nullptr;
 }
 
+void ElementVisibilityObserver::deliverObservationsForTesting() {
+  m_intersectionObserver->deliver();
+}
+
 DEFINE_TRACE(ElementVisibilityObserver) {
   visitor->trace(m_element);
   visitor->trace(m_intersectionObserver);