Update poster code as per https://html.spec.whatwg.org/#show-poster-flag

Source/core/html/HTMLMediaElement.h

Index: Source/core/html/HTMLMediaElement.h
diff --git a/Source/core/html/HTMLMediaElement.h b/Source/core/html/HTMLMediaElement.h
index 6cdc6faf30d5ba317298a5e871fc2c74c89f502e..1c313e8bbf87e3f91e45308eee0b631e5b8f88e7 100644
--- a/Source/core/html/HTMLMediaElement.h
+++ b/Source/core/html/HTMLMediaElement.h
@@ -281,8 +281,8 @@ protected:
     virtual KURL posterImageURL() const { return KURL(); }
 
     enum DisplayMode { Unknown, Poster, Video };
-    DisplayMode displayMode() const { return m_displayMode; }
-    virtual void setDisplayMode(DisplayMode mode) { m_displayMode = mode; }
+    virtual DisplayMode displayMode() const { return Unknown; }

[philipj_slow, 2015/08/18 08:24:48]

Do these need to be virtual and on HTMLMediaElement at all? If there are very
few call sites, maybe they can just cast to HTMLVideoElement instead?

[Srirama, 2015/08/18 08:58:26]

But our intention is to make sure these functions won't execute incase of Audio
element. So if we move these functions completely to HTMLVideoElement, it will
again be same as the current code right. Or you want me to move them and then
add VideoElement check at the call site in HTMLMediaElement?

[philipj_slow, 2015/08/18 09:10:28]

It looks like the displayMode() getter is only used in HTMLVideoElement, so at
least that doesn't need to be virtual.

There are three call sites to setDisplayMode() in HTMLMediaElement, and I think
it almost corresponds to https://html.spec.whatwg.org/#show-poster-flag in the
spec.

So here's an idea: introduce the show poster flag and update it per spec, then
perhaps the setShowPoster() call can be virtual so that HTMLVideoElement does
something extra before or after if needed.

+    virtual void setDisplayMode(DisplayMode mode) { }
 
     void setControllerInternal(MediaController*);
 
@@ -508,8 +508,6 @@ private:
     OwnPtr<WebMediaPlayer> m_webMediaPlayer;
     WebLayer* m_webLayer;
 
-    DisplayMode m_displayMode;
-
     RefPtrWillBeMember<HTMLMediaSource> m_mediaSource;
 
     // Cached time value. Only valid when ready state is HAVE_METADATA or