Always show "exit fullscreen" button.

Source/core/html/shadow/MediaControls.cpp

 /*
  * Copyright (C) 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2011, 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 166 matching lines @@
     m_timeline->setPosition(mediaElement().currentTime());
 
     if (!mediaElement().hasAudio())
         m_volumeSlider->hide();
     else
         m_volumeSlider->show();
     updateVolume();
 
     refreshClosedCaptionsButtonVisibility();
 
-    if (mediaElement().hasVideo() && fullscreenIsSupported(document()))
+    // Unconditionally allow the user to exit fullscreen if we are in it
+    // now.  Especially on android, when we might not yet know if
+    // fullscreen is supported, we sometimes guess incorrectly and show
+    // the button earlier, and we don't want to remove it here if the
+    // user chose to enter fullscreen.  crbug.com/500732 .
+    if ((mediaElement().hasVideo() && fullscreenIsSupported(document()))
+        || mediaElement().isFullscreen())
         m_fullScreenButton->show();
     else
         m_fullScreenButton->hide();
 
     refreshCastButtonVisibility();
     makeOpaque();
 }
 
 LayoutObject* MediaControls::layoutObjectForTextTrackLayout()
 {
@@ skipping 335 matching lines @@
     visitor->trace(m_toggleClosedCaptionsButton);
     visitor->trace(m_fullScreenButton);
     visitor->trace(m_durationDisplay);
     visitor->trace(m_enclosure);
     visitor->trace(m_castButton);
     visitor->trace(m_overlayCastButton);
     HTMLDivElement::trace(visitor);
 }
 
 }