LibWeb: Add a getter for HTMLMediaElement's current playback position

This will be needed by the layout node, which may change what is painted
when the position of the frame image is not the same as the element's
current time.

Userland/Libraries/LibWeb/HTML/HTMLMediaElement.h

@@ -63,6 +63,8 @@ public:
 
     double current_time() const;
     void set_current_time(double);
+
+    double current_playback_position() const { return m_current_playback_position; }
     void set_current_playback_position(double);
 
     double duration() const;