Implement FullScreen using top layer.

third_party/WebKit/Source/core/css/fullscreen.css

-:-webkit-full-screen {
-    background-color: white;
-    z-index: 2147483647 !important;
-}
-
-:root:-webkit-full-screen-ancestor {
-    overflow: hidden !important;
-}
-
-:-webkit-full-screen-ancestor:not(iframe) {
-    z-index: auto !important;
-    position: static !important;
-    opacity: 1 !important;
-    transform: none !important;
-    -webkit-mask: none !important;
-    clip: none !important;
-    -webkit-filter: none !important;
-    transition: none !important;
-    -webkit-box-reflect: none !important;
-    -webkit-perspective: none !important;
-    -webkit-transform-style: flat !important;
-}
-
-video:-webkit-full-screen, audio:-webkit-full-screen {
-    background-color: transparent !important;
-    position: relative !important;
+:not(:root):-webkit-full-screen {
+    position: fixed !important;
+    top: 0 !important;
+    right: 0 !important;
+    bottom: 0 !important;
     left: 0 !important;
-    top: 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;
-    flex: 1 !important;
-    display: block !important;
+    object-fit: contain !important;
     transform: none !important;
 }
 
-img:-webkit-full-screen {
-    width: auto;
-    height: 100%;
-    max-width: 100%;
+iframe:-webkit-full-screen {
+    border: none !important;
+    padding: 0 !important;
 }
 
-iframe:-webkit-full-screen {
-    margin: 0 !important;
-    padding: 0 !important;
-    border: 0 !important;
-    position: fixed !important;
-    min-width: 0 !important;
-    max-width: none !important;
-    min-height: 0 !important;
-    max-height: none !important;
-    width: 100% !important;
-    height: 100% !important;
-    left: 0 !important;
-    top: 0 !important;
+:not(:root):-webkit-full-screen::backdrop {
+    position: fixed;
+    top: 0;
+    right: 0;
+    bottom: 0;
+    left: 0;
+    background: black;
 }
+
+/* Anything below are extensions over what the Fullscreen API (29 September 2015) mandates. */
+
+/* This is required for https://crbug.com/356282 */
+video:-webkit-full-screen, audio:-webkit-full-screen {
+    transform: none !important;

[philipj_slow, 2015/10/01 15:00:23]

It should be possible to remove this now that it's part of the 
:not(:root):-webkit-full-screen block. An audio or video element as the root
element shouldn't be much of a concern :)

+}
+
+/* This prevents video from overflowing the viewport in
+   virtual/android/fullscreen/video-scrolled-iframe.html
+   FIXME: We should remove this (see crbug.com/441890). */
+:root:-webkit-full-screen-ancestor {

[philipj_slow, 2015/10/01 15:00:23]

It's probably a good idea to leave this until a later CL, but doesn't the top
layer rewrite make the ancestor pseudo-class unnecessary? The code for keeping
track of it is a bit fragile so it sure would be nice to get rid of.

+    overflow: hidden;
+}