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

third_party/WebKit/Source/core/dom/ElementVisibilityObserver.h

Index: third_party/WebKit/Source/core/dom/ElementVisibilityObserver.h
diff --git a/third_party/WebKit/Source/core/dom/ElementVisibilityObserver.h b/third_party/WebKit/Source/core/dom/ElementVisibilityObserver.h
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+// 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.
+
+#ifndef ElementVisibilityObserver_h
+#define ElementVisibilityObserver_h
+
+#include "core/dom/IntersectionObserver.h"
+#include "platform/heap/Heap.h"
+#include "platform/heap/Member.h"
+
+namespace blink {
+
+class Element;
+
+// ElementVisibilityObserver is a helper class to be used to track the
+// visibility of an Element in the viewport. Creating an
+// ElementVisibilityObserver is a no-op with regards to CPU cycle. The observing
+// has be started by calling |start()| and can be stopped with |stop()|.
+// When creating an instance, the caller will have to pass a callback taking
+// a boolean as an argument. The boolean will be the new visibility state.
+// The ElementVisibilityObserver is implemented on top of IntersectionObserver.
+// It is a layer meant to simplify the usage for C++ Blink code checking for the
+// visibility of an element.
+class ElementVisibilityObserver final : public GarbageCollectedFinalized<ElementVisibilityObserver> {
+    WTF_MAKE_NONCOPYABLE(ElementVisibilityObserver);
+public:
+    using VisibilityCallback = Function<void(bool), WTF::SameThreadAffinity>;
+
+    ElementVisibilityObserver(Element*, std::unique_ptr<VisibilityCallback>);
+    virtual ~ElementVisibilityObserver();
+
+    void start();
+    void stop();
+
+    DECLARE_VIRTUAL_TRACE();
+
+private:
+    class ElementVisibilityCallback;
+
+    void onVisibilityChanged(const HeapVector<Member<IntersectionObserverEntry>>&);
+
+    Member<Element> m_element;
+    Member<IntersectionObserver> m_intersectionObserver;
+    std::unique_ptr<VisibilityCallback> m_callback;
+};
+
+} // namespace blink
+
+#endif // ElementVisibilityObserver_h