Implement "potentially playing" for media elements per spec

Source/core/html/HTMLMediaElement.cpp

Index: Source/core/html/HTMLMediaElement.cpp
diff --git a/Source/core/html/HTMLMediaElement.cpp b/Source/core/html/HTMLMediaElement.cpp
index 873298fe8ed0e570630e8a8456f86c8aa3b83193..0a38e3928a3df31ae88e9730f48396c1b913ae0a 100644
--- a/Source/core/html/HTMLMediaElement.cpp
+++ b/Source/core/html/HTMLMediaElement.cpp
@@ -336,7 +336,6 @@ HTMLMediaElement::HTMLMediaElement(const QualifiedName& tagName, Document& docum
     , m_defaultPlaybackRate(1.0f)
     , m_networkState(NETWORK_EMPTY)
     , m_readyState(HAVE_NOTHING)
-    , m_readyStateMaximum(HAVE_NOTHING)
     , m_volume(1.0f)
     , m_lastSeekTime(0)
     , m_previousProgressTime(std::numeric_limits<double>::max())
@@ -796,7 +795,6 @@ void HTMLMediaElement::prepareForLoad()
 
         // 4.4 - If readyState is not set to HAVE_NOTHING, then set it to that state.
         m_readyState = HAVE_NOTHING;
-        m_readyStateMaximum = HAVE_NOTHING;
 
         // 4.5 - If the paused attribute is false, then set it to true.
         m_paused = true;
@@ -1802,9 +1800,6 @@ void HTMLMediaElement::setReadyState(ReadyState state)
             m_readyState = HAVE_CURRENT_DATA;
     }
 
-    if (oldState > m_readyStateMaximum)
-        m_readyStateMaximum = oldState;
-
     if (m_networkState == NETWORK_EMPTY)
         return;
 
@@ -3382,11 +3377,27 @@ PassRefPtrWillBeRawPtr<TimeRanges> HTMLMediaElement::seekable() const
 
 bool HTMLMediaElement::potentiallyPlaying() const
 {
-    // "pausedToBuffer" means the media engine's rate is 0, but only because it had to stop playing
-    // when it ran out of buffered data. A movie is this state is "potentially playing", modulo the
-    // checks in couldPlayIfEnoughData().
-    bool pausedToBuffer = m_readyStateMaximum >= HAVE_FUTURE_DATA && m_readyState < HAVE_FUTURE_DATA;
-    return (pausedToBuffer || m_readyState >= HAVE_FUTURE_DATA) && couldPlayIfEnoughData() && !isBlockedOnMediaController();
+    // A media element is said to be potentially playing when its paused attribute is false, the
+    // element has not ended playback, playback has not stopped due to errors, the element either
+    // has no current media controller or has a current media controller but is not blocked on its
+    // media controller, and the element is not a blocked media element.
+
+    if (paused())
+        return false;
+
+    if (endedPlayback())
+        return false;
+
+    if (stoppedDueToErrors())
+        return false;
+
+    if (isBlockedOnMediaController())
+        return false;
+
+    if (isBlocked())
+        return false;
+
+    return true;
 }
 
 bool HTMLMediaElement::couldPlayIfEnoughData() const