Adjust to webkitSupportsFullscreen and other APIs being removed from Chromium

[tguilbert-google]

Chromium is looking to remove the deprecated HTMLVideoElement specific prefixed fullscreen APIs (link to discussion).

Specifically, this means that the following APIs will be removed from Chromium:

- readonly attribute boolean webkitSupportsFullscreen;
- readonly attribute boolean webkitDisplayingFullscreen;
- void webkitEnterFullscreen();
- void webkitExitFullscreen();
- void webkitEnterFullScreen(); // Note the different capitalization of the "S" in FullScreen.
- void webkitExitFullScreen(); // Note the different capitalization of the "S" in FullScreen.

We have seen that some sites using Clappr do check for or use webkitSupportsFullscreen.

Running chrome with the --disable-blink-features=PrefixedVideoFullscreen should be enough to confirm that Clappr properly handles the removal of those flags.

Would it be possible to see if you are running into any issues while using that flag? Thank you!