Use intersection observer to control frame throttling

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

Index: third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
diff --git a/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp b/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
index 7509dcf7051d0c111c615918360c489eed7e098e..32ef04c78978b5f0856a4812c1ff8499585d1e5d 100644
--- a/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
+++ b/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
@@ -203,7 +203,8 @@ IntersectionObserver::IntersectionObserver(
       m_topMargin(Fixed),
       m_rightMargin(Fixed),
       m_bottomMargin(Fixed),
-      m_leftMargin(Fixed) {
+      m_leftMargin(Fixed),
+      m_initialState(InitialState::kHidden) {
   switch (rootMargin.size()) {
     case 0:
       break;
@@ -291,6 +292,11 @@ void IntersectionObserver::observe(Element* target,
     return;
   }
 
+  if (m_initialState == InitialState::kAuto) {
+    for (auto& observation : m_observations)
+      observation->setLastThresholdIndex(std::numeric_limits<unsigned>::max());
+  }
+
   if (!rootFrame)
     return;
   if (FrameView* rootFrameView = rootFrame->view())
@@ -345,6 +351,11 @@ void IntersectionObserver::removeObservation(
   m_observations.remove(&observation);
 }
 
+void IntersectionObserver::setInitialState(InitialState initialState) {
+  DCHECK(m_observations.isEmpty());
+  m_initialState = initialState;
+}
+
 HeapVector<Member<IntersectionObserverEntry>> IntersectionObserver::takeRecords(
     ExceptionState& exceptionState) {
   HeapVector<Member<IntersectionObserverEntry>> entries;