Revert of Disable the HTMLVideoElement-specific prefixed fullscreen API (2nd attempt) (https://codereview.chromium.org/251873002/)

Reason for revert:
Wait with removal until the unprefixed Fullscreen API has been shipped.

Original issue's description:
> Disable the HTMLVideoElement-specific prefixed fullscreen API (2nd attempt)
> 
> BUG=346236
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=172693

Review URL: https://codereview.chromium.org/328303003

git-svn-id: svn://svn.chromium.org/blink/trunk@176144 bbb929c8-8fbe-4397-9dbb-9b2b20218538