Start autoplay muted videos with autoplay attribute when they are visible.

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 26fe2ecfdbbe9437d9efcd4d505b9d13e67f5e28..f6e9e524690b758eeca4f983302ccb14e85e6fa3 100644
--- a/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
+++ b/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
@@ -179,12 +179,19 @@ void IntersectionObserver::observe(Element* target, ExceptionState& exceptionSta
         return;
     }
 
+    observe(target);
+}
+
+void IntersectionObserver::observe(Element* target)
+{
+    if (!m_root)
+        return;
+
     if (!target || m_root.get() == target)
         return;
 
     if (target->ensureIntersectionObserverData().getObservationFor(*this))
         return;
-
     bool shouldReportRootBounds = false;
     bool isDOMDescendant = false;
     LocalFrame* targetFrame = target->document().frame();
@@ -222,6 +229,14 @@ void IntersectionObserver::unobserve(Element* target, ExceptionState& exceptionS
         return;
     }
 
+    unobserve(target);
+}
+
+void IntersectionObserver::unobserve(Element* target)
+{
+    if (!m_root)
+        return;
+
     if (!target || !target->intersectionObserverData())
         return;
     // TODO(szager): unobserve callback
@@ -340,7 +355,6 @@ unsigned IntersectionObserver::firstThresholdGreaterThan(float ratio) const
 
 void IntersectionObserver::deliver()
 {
-
     if (m_entries.isEmpty())
         return;