Remove TreatNullAs=NullString for media interfaces

Source/core/html/HTMLVideoElement.idl

 /*
  * Copyright (C) 2007, 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 12 matching lines @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 [
     RuntimeEnabled=Media
 ] interface HTMLVideoElement : HTMLMediaElement {
     [Reflect] attribute unsigned long width;
     [Reflect] attribute unsigned long height;
     readonly attribute unsigned long videoWidth;
     readonly attribute unsigned long videoHeight;
-    [Reflect, TreatNullAs=NullString, URL] attribute DOMString poster;
+    [Reflect, URL] attribute DOMString poster;
 
     [MeasureAs=PrefixedVideoSupportsFullscreen] readonly attribute boolean webkitSupportsFullscreen;
     [MeasureAs=PrefixedVideoDisplayingFullscreen] readonly attribute boolean webkitDisplayingFullscreen;
 
     [MeasureAs=PrefixedVideoEnterFullscreen, RaisesException, PerWorldBindings, ActivityLogging=AccessForAllWorlds] void webkitEnterFullscreen();
     [MeasureAs=PrefixedVideoExitFullscreen] void webkitExitFullscreen();
 
     // Note the different capitalization of the "S" in FullScreen.
     [MeasureAs=PrefixedVideoEnterFullScreen, ImplementedAs=webkitEnterFullscreen, RaisesException, PerWorldBindings, ActivityLogging=AccessForAllWorlds] void webkitEnterFullScreen();
     [MeasureAs=PrefixedVideoExitFullScreen, ImplementedAs=webkitExitFullscreen] void webkitExitFullScreen();
 
     // The number of frames that have been decoded and made available for
     // playback.
     [MeasureAs=PrefixedVideoDecodedFrameCount] readonly attribute unsigned long webkitDecodedFrameCount;
 
     // The number of decoded frames that have been dropped by the player
     // for performance reasons during playback.
     [MeasureAs=PrefixedVideoDroppedFrameCount] readonly attribute unsigned long webkitDroppedFrameCount;
 };