Implement autoplay gesture override experiment.

Source/core/layout/LayoutMedia.cpp

 /*
  * Copyright (C) 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 17 matching lines @@
 #include "core/layout/LayoutMedia.h"
 
 #include "core/html/HTMLMediaElement.h"
 #include "core/html/shadow/MediaControls.h"
 #include "core/layout/LayoutView.h"
 
 namespace blink {
 
 LayoutMedia::LayoutMedia(HTMLMediaElement* video)
     : LayoutImage(video)
+    , m_registeredForPositionChangeNotifications(false)
+    , m_wantPositionChangeNotifications(false)
 {
     setImageResource(LayoutImageResource::create());
 }
 
 LayoutMedia::~LayoutMedia()
 {
 }
 
 HTMLMediaElement* LayoutMedia::mediaElement() const
 {
@@ skipping 64 matching lines @@
     if (child->node()->isTextTrackContainer())
         return true;
 
     return false;
 }
 
 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