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

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
new file mode 100644
index 0000000000000000000000000000000000000000..915e57574ed9fb2220d94519bb4a3b052f60a86c
--- /dev/null
+++ b/third_party/WebKit/Source/core/dom/ElementVisibilityObserver.cpp
@@ -0,0 +1,51 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/dom/ElementVisibilityObserver.h"
+
+#include "core/dom/Element.h"
+#include "core/dom/IntersectionObserverEntry.h"
+#include "wtf/Functional.h"
+
+namespace blink {
+
+ElementVisibilityObserver::ElementVisibilityObserver(Element* element, std::unique_ptr<VisibilityCallback> callback)
+    : m_element(element)
+    , m_callback(std::move(callback))
+{
+}
+
+ElementVisibilityObserver::~ElementVisibilityObserver() = default;
+
+void ElementVisibilityObserver::start()
+{
+    DCHECK(!m_intersectionObserver);
+    m_intersectionObserver = IntersectionObserver::create(
+        Vector<Length>(), Vector<float>({std::numeric_limits<float>::min()}), &m_element->document(),
+        WTF::bind<const HeapVector<Member<IntersectionObserverEntry>>&>(&ElementVisibilityObserver::onVisibilityChanged, WeakPersistentThisPointer<ElementVisibilityObserver>(this)));
+    DCHECK(m_intersectionObserver);
+    m_intersectionObserver->observe(m_element);
+}
+
+void ElementVisibilityObserver::stop()
+{
+    DCHECK(m_intersectionObserver);
+
+    m_intersectionObserver->unobserve(m_element);
+    m_intersectionObserver = nullptr;
+}
+
+DEFINE_TRACE(ElementVisibilityObserver)
+{
+    visitor->trace(m_element);
+    visitor->trace(m_intersectionObserver);
+}
+
+void ElementVisibilityObserver::onVisibilityChanged(const HeapVector<Member<IntersectionObserverEntry>>& entries)
+{
+    bool isVisible = entries.last()->intersectionRatio() > 0.f;
+    (*m_callback.get())(isVisible);
+}
+
+} // namespace blink