Reland r165710 "Replace RenderFullScreen with top layer"

Source/core/css/fullscreen.css

Index: Source/core/css/fullscreen.css
diff --git a/Source/core/css/fullscreen.css b/Source/core/css/fullscreen.css
index 5ea5f06c2f071bf792657394e4c80fcec9e05b92..15d0a529a5e5cce0bd26ccfe3578baea18b3c5fc 100644
--- a/Source/core/css/fullscreen.css
+++ b/Source/core/css/fullscreen.css
@@ -1,6 +1,27 @@
 :-webkit-full-screen {
     background-color: white;
-    z-index: 2147483647 !important;
+}
+
+:not(:root):-webkit-full-screen {
+    position: fixed;
+    top: 0;
+    right: 0;
+    bottom: 0;
+    left: 0;
+    margin: 0;
+    box-sizing: border-box;
+    width: 100%;
+    height: 100%;
+    object-fit: contain;
+}
+
+:-webkit-full-screen::backdrop {
+    position: fixed;
+    top: 0;
+    right: 0;
+    bottom: 0;
+    left: 0;
+    background: black;
 }
 
 :root:-webkit-full-screen-document:not(:-webkit-full-screen), :root:-webkit-full-screen-ancestor {