Implement autoplay gesture override experiment.

Source/core/layout/LayoutMedia.cpp

Index: Source/core/layout/LayoutMedia.cpp
diff --git a/Source/core/layout/LayoutMedia.cpp b/Source/core/layout/LayoutMedia.cpp
index 374c4b1889cb25e1e8043d9c099085f857c5906d..664ab2055fdddaf6043b83c4e7a6ff226197b390 100644
--- a/Source/core/layout/LayoutMedia.cpp
+++ b/Source/core/layout/LayoutMedia.cpp
@@ -35,6 +35,8 @@ namespace blink {
 
 LayoutMedia::LayoutMedia(HTMLMediaElement* video)
     : LayoutImage(video)
+    , m_registeredForPositionChangeNotifications(false)
+    , m_wantPositionChangeNotifications(false)
 {
     setImageResource(LayoutImageResource::create());
 }
@@ -119,4 +121,61 @@ void LayoutMedia::paintReplaced(const PaintInfo&, const LayoutPoint&)
 {
 }
 
+void LayoutMedia::willBeDestroyed()
+{
+    updatePositionChangeRegistration(false);
+    LayoutImage::willBeDestroyed();
+}
+
+void LayoutMedia::insertedIntoTree()
+{
+    LayoutImage::insertedIntoTree();
+
+    // Note that if we don't want them and aren't registered, then this
+    // will do nothing.
+    updatePositionChangeRegistration(m_wantPositionChangeNotifications);
+}
+
+void LayoutMedia::willBeRemovedFromTree()
+{
+    // Note that if we don't want them and/or aren't registered, then this
+    // will do nothing.
+    updatePositionChangeRegistration(false);
+
+    LayoutImage::willBeRemovedFromTree();
+}
+
+void LayoutMedia::notifyPositionMayHaveChanged()
+{
+    // Tell our element about it.
+    HTMLMediaElement* element = mediaElement();
+    if (element)

[ojan, 2015/09/01 20:20:11]

Nit: Typical blink style is to do the following, since there's no chance of
human error in the case where you need to include the type (i.e. the compiler
will yell at you if you accidentally type == instead of =):
if (HTMLMediaElement* element = mediaElement())

[liberato (no reviews please), 2015/09/04 06:49:46]

never thought of that, thanks!

+        element->notifyPositionMayHaveChanged();

[ojan, 2015/09/01 20:20:11]

How about having the LayoutMedia store a bit as to whether the video is visible?
That way you only have to notify when the video visibility changes and you don't
need to start timers every time the video moves. Adding timers has a performance
impact, so it's best to start them only when you actually need them.

In this world, HTMLMediaElement doesn't need to store a rect, it would just
store a bool as to whether it's visible or not.

[liberato (no reviews please), 2015/09/04 06:49:46]

i implemented the logic as you suggest.  however, i put it in
AutoplayExperimentHelper, so that it's all in one place.

+}
+
+void LayoutMedia::updatePositionChangeRegistration(bool doRegister)
+{
+    // If we don't have a view, then take no action.
+    if (!view())
+        return;
+
+    // If our registration state matches the incoming state, then no work
+    // is needed.
+    if (doRegister == m_registeredForPositionChangeNotifications)
+        return;
+
+    if (doRegister)
+        view()->registerMediaForPositionChangeNotification(this);
+    else
+        view()->unregisterMediaForPositionChangeNotification(this);
+
+    m_registeredForPositionChangeNotifications = doRegister;
+}
+
+void LayoutMedia::setRequestPositionUpdates(bool want)
+{
+    m_wantPositionChangeNotifications = want;
+    updatePositionChangeRegistration(m_wantPositionChangeNotifications);
+}
+
 } // namespace blink