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..260f68bfba19a1ad02566475ce31d43ef27a4aa7 100644
--- a/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
+++ b/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
@@ -345,6 +345,12 @@ void IntersectionObserver::removeObservation(
   m_observations.remove(&observation);
 }
 
+void IntersectionObserver::setInitialState(InitialState initialState) {

[ojan, 2016/10/31 19:37:09]

This will happen to work for ElementVisibilityObserver because it only calls
observe once before setInitialState, but if any code called observe after
calling setInitialState, it wouldn't work.

I think what we want here is instead to:
1. Store a member on the IntersectionObserver that says the initial state is
Auto.
2. In observe, call setLastThresholdIndex when initial state == Auto.
3. DCHECK in setInitialState that observations() is empty. Then no need to loop
over them obviously. 
4. Put the setInitialState call after the IntersectionObserver::create in
ElementVisibilityObserver.

That way, future code that call setInitialState + observe will work right.

[Sami, 2016/11/01 10:44:48]

Thanks, I think this will help to avoid some surprises in the future. The only
detail I had to change was that the default initial state should be kHidden to
preserve the current behavior of IntersectionObserver.

+  DCHECK_EQ(initialState, InitialState::AUTO);
+  for (auto& observation : observations())
+    observation->setLastThresholdIndex(std::numeric_limits<unsigned>::max());
+}
+
 HeapVector<Member<IntersectionObserverEntry>> IntersectionObserver::takeRecords(
     ExceptionState& exceptionState) {
   HeapVector<Member<IntersectionObserverEntry>> entries;