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

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

+// 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)
+{
+    DCHECK_EQ(entries.size(), 1u);

[szager1, 2016/06/21 21:18:05]

This is a bad assumption; observations are taken every frame, but may be
delivered up to 100ms later.  If multiple visibility events happened during that
100ms, they will be bundled here.

The entries are ordered by the time they occurred, earliest to latest, so you
can get rid of the DCHECK and just:

bool isVisible = entries.last()->intersectionRatio() > 0.f;

[mlamouri (slow - plz ping), 2016/06/21 21:25:50]

Good to know! These small things comforts me in having this
ElementVisibilityObserver layer :)

+
+    bool isVisible = entries[0]->intersectionRatio() > 0.f;
+    (*m_callback.get())(isVisible);
+}
+
+} // namespace blink