chrome: Support customization of "exit fullscreen" bubble

[magreenblatt]

Is your feature request related to a problem? Please describe. When entering fullscreen mode, Chrome shows a transient bubble with contents "Press [key] to exit full screen". It would be nice if the appearance of this bubble could be customized.

Describe the solution you'd like It should be possible to customize the look/feel (colors, font, text) of this bubble. It might also be nice to use the same [key] to exit both browser fullscreen and HTML5 video fullscreen (currently F11 and Esc respectively on Windows).

Additional context Related to issue #3571 where we standardized fullscreen callbacks.

[magreenblatt]

It might also be nice to use the same [key] to exit both browser fullscreen and HTML5 video fullscreen

That's probably not a good idea, since it's possible to enter browser fullscreen followed by HTML5 video fullscreen, and then exit them in reverse order with different keys.

[magreenblatt]

On MacOS, the "exit fullscreen" bubble does not display when initially entering fullscreen mode (browser or video). It does display if you:

1. Enter browser fullscreen via the Chrome toolbar fullscreen button
2. Enter video fullscreen (shows the video fullscreen exit key)
3. Exit video fullscreen to return to browser fullscreen (shows the browser fullscreen exit key)

If in step 1 you instead enter browser fullscreen via the traffic light window button then it sometimes does not show the browser fullscreen exit key in step 3 (seems to depend on whether the traffic light button or the Chrome toolbar button is used first after application launch).

This is likely a bug, and the "exit fullscreen" bubble should display in all cases.

[magreenblatt]

The inconsistent fullscreen bubble display on MacOS will be fixed in #3597.