Always show "exit fullscreen" button.

Source/core/html/shadow/MediaControls.cpp

Index: Source/core/html/shadow/MediaControls.cpp
diff --git a/Source/core/html/shadow/MediaControls.cpp b/Source/core/html/shadow/MediaControls.cpp
index 43d22a4cf8f2bc617013e90ffb0185038d8b74aa..17d1096113fffeb9acbde1c6aaa10e83a405b48e 100644
--- a/Source/core/html/shadow/MediaControls.cpp
+++ b/Source/core/html/shadow/MediaControls.cpp
@@ -184,7 +184,13 @@ void MediaControls::reset()
 
     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();