Expose the poster image URL to WebMediaPlayer subclasses

public/platform/WebMediaPlayer.h

Index: public/platform/WebMediaPlayer.h
diff --git a/public/platform/WebMediaPlayer.h b/public/platform/WebMediaPlayer.h
index 0d83c8caf30f028e5080265cfdaba66f5cd6b0fa..759f986a728fde194c5a8944004a82d52552bf20 100644
--- a/public/platform/WebMediaPlayer.h
+++ b/public/platform/WebMediaPlayer.h
@@ -154,6 +154,8 @@ public:
     virtual MediaKeyException cancelKeyRequest(const WebString& keySystem, const WebString& sessionId) { return MediaKeyExceptionKeySystemNotSupported; }
     virtual void setContentDecryptionModule(WebContentDecryptionModule* cdm) { }
 
+    virtual void setPoster(const WebURL& poster) { }
+
     // Instruct WebMediaPlayer to enter/exit fullscreen.
     virtual void enterFullscreen() { }
     virtual void exitFullscreen() { }