Move iframe border/padding to the standard Fullscreen UA style sheet

Source/core/css/fullscreen.css

 /*
  * User-agent level style sheet defaults for the Fullscreen API
  * https://fullscreen.spec.whatwg.org/#user-agent-level-style-sheet-defaults
  *
  * FIXME: Use the :fullscreen pseudo-class instead of :-webkit-full-screen.
  * (see crbug.com/402378)
  */
 
 :not(:root):-webkit-full-screen {
     position: fixed !important;
     top: 0 !important;
     right: 0 !important;
     bottom: 0 !important;
     left: 0 !important;
     margin: 0 !important;
     box-sizing: border-box !important;
     min-width: 0 !important;
     max-width: none !important;
     min-height: 0 !important;
     max-height: none !important;
     width: 100% !important;
     height: 100% !important;
     object-fit: contain !important;
     transform: none !important;
 }
 
 iframe:-webkit-full-screen {
-    border: none;
-}
-
-/* Anything below are extensions over what the Fullscreen API mandates. */
-
-/* The border and padding of fullscreen iframes are removed, as there are sites
-   that set border or padding on iframes with the apparent expectation that they
-   will be ignored in fullscreen (see crbug.com/468292 and crbug.com/469133).
-   TODO(philipj): Remove this or merge into the above block when the spec bug
-   has been resolved: https://www.w3.org/Bugs/Public/show_bug.cgi?id=28332 */
-iframe:-webkit-full-screen {
     border: none !important;
     padding: 0 !important;
 }
 
+/* Anything below are extensions over what the Fullscreen API mandates. */
+
 /* FIXME: Remove these rules when moving Fullscreen to top layer.
    (see crbug.com/240576) */
 :-webkit-full-screen {
     background-color: white;
     z-index: 2147483647 !important;
 }
 
 video:-webkit-full-screen, audio:-webkit-full-screen {
     background-color: transparent !important;
 }
@@ skipping 11 matching lines @@
     -webkit-perspective: none !important;
     -webkit-transform-style: flat !important;
 }
 
 /* This prevents video from overflowing the viewport in
    virtual/android/fullscreen/video-scrolled-iframe.html
    FIXME: We should remove this (see crbug.com/441890). */
 :-webkit-full-screen-ancestor {
     overflow: hidden !important;
 }