Fix (UI): Check fullscreenEnabled in dblclick

Our "double tap on the video to enter fullscreen" functionality was
not checking to see if fullscreen was enabled or not first, which
would lead to the player erroring if the user double-clicks the
video on a platform that does not support fullscreen.

Fixes shaka-project#3441

Change-Id: Id96f783a265e0e05fba93cae2da3904bf1747c9b

ui/controls.js

@@ -687,7 +687,7 @@ shaka.ui.Controls = class extends shaka.util.FakeEventTarget {
     });
 
     this.eventManager_.listen(this.controlsContainer_, 'dblclick', () => {
-      if (this.config_.doubleClickForFullscreen) {
+      if (this.config_.doubleClickForFullscreen && document.fullscreenEnabled) {
         this.toggleFullScreen();
       }
     });