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>({0.f}), &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);
+
+    bool isVisible = entries[0]->intersectionRatio() > 0.f;

[szager1, 2016/06/17 07:32:36]

Unfortunately, this will not work.  IntersectionObserver treats a zero threshold
specially: it signifies a transition between two states: "not intersecting at
all", and "intersecting by any amount, including a zero-area intersection when
root and target have a coincident corner or edge."

So intersectionRatio() can be zero even if the notification indicates a
transition from not-intersecting to intersecting.

This is a known gotcha with IntersectionObserver, and we are open to ways to
make this less obscure.  For the time being, you might be better off creating an
IntersectionObserver with Vector<float>({std::numeric_limits<float>::FLT_MIN})
for the thresholds argument.  If you do that, then this line will work as
intended.

Otherwise, you'll have to keep a boolean state variable around, initialized to
"not intersecting", and flip it on every notification.

[mlamouri (slow - plz ping), 2016/06/17 11:52:04]

Done.

+    (*m_callback.get())(isVisible);
+}
+
+} // namespace blink