Restore the HTMLVideoElement-specific prefixed fullscreen API behind a flag

LayoutTests/fullscreen/full-screen-iframe-legacy.html

Index: LayoutTests/fullscreen/full-screen-iframe-legacy.html
diff --git a/LayoutTests/fullscreen/full-screen-iframe-legacy.html b/LayoutTests/fullscreen/full-screen-iframe-legacy.html
index 187a3397f31bd4f8a72eb257be5e349af132a04d..6a3bb524f23052417b1df9e6c95ca15f3293509b 100644
--- a/LayoutTests/fullscreen/full-screen-iframe-legacy.html
+++ b/LayoutTests/fullscreen/full-screen-iframe-legacy.html
@@ -1,7 +1,6 @@
 <p>Test for <a href="https://bugs.webkit.org/show_bug.cgi?id=61461">bug 61461</a>: 
-Handle entering full screen security restrictions.
-Note that the original pass condition has been reversed.</p>
-<p>To test manually, click the video "full screen" button - the page should NOT enter full screen mode.</p>
+Handle entering full screen security restrictions</p>
+<p>To test manually, click the video "full screen" button - the page should enter full screen mode.</p>
 <script src="full-screen-test.js"></script>
 <script src="../media/media-file.js"></script>
 <script>
@@ -11,15 +10,16 @@ function canplaythrough() {
 
     runWithKeyDown(function() {
         var video = frame.contentDocument.getElementsByTagName('video')[0];
-        video.webkitRequestFullscreen();
+        video.webkitEnterFullScreen();
     });
 }
 
 function runTest() {
     var frame = document.getElementById('frame');
 
-    waitForEvent(frame.contentDocument, 'webkitfullscreenerror', function() {
-        endTest();
+    waitForEvent(frame.contentDocument, 'webkitfullscreenchange', function() {
+            test("document.getElementById('frame').contentDocument.width==document.width")
+            endTest();
     });
 
     var video = frame.contentDocument.getElementsByTagName('video')[0];