Add tests while FullscreenUnprefixed flag is turned off.

third_party/WebKit/LayoutTests/shadow-dom/fullscreen-element-in-shadow-simple.html

Index: third_party/WebKit/LayoutTests/shadow-dom/fullscreen-element-in-shadow-simple.html
diff --git a/third_party/WebKit/LayoutTests/shadow-dom/fullscreen-element-in-shadow-simple.html b/third_party/WebKit/LayoutTests/shadow-dom/fullscreen-element-in-shadow-simple.html
index ecec654d58a77f894ee4cca7c89199cb1f446046..8a5997f7c79b7e669260001a939986665515c797 100644
--- a/third_party/WebKit/LayoutTests/shadow-dom/fullscreen-element-in-shadow-simple.html
+++ b/third_party/WebKit/LayoutTests/shadow-dom/fullscreen-element-in-shadow-simple.html
@@ -35,6 +35,16 @@ async_test((test) => {
         assert_equals(document.webkitCurrentFullScreenElement, host0);
 
         assert_equals(host0.shadowRoot.fullscreenElement, canvas);
+
+        // TODO(kochi): The 'retargeting' behavior is controlled by
+        // 'FullscreenUnprefixed' runtime flag dynamically.  Until the
+        // flag is turned on, document.webkitFullscreenElement leaks the
+        // fullscreen element under shadow roots.
+        window.internals.runtimeFlags.fullscreenUnprefixedEnabled = false;
+        assert_false(window.internals.runtimeFlags.fullscreenUnprefixedEnabled);
+        assert_equals(document.webkitFullscreenElement, canvas);
+        assert_equals(document.webkitCurrentFullScreenElement, canvas);
+
         document.exitFullscreen();
         test.done();
     });