This bug is to track a known issue that is not a regression, or a blocker. 

It would be a better experience to seamlessly transition between full-screen 2D and full-screen VR. Currently the fullscreen state is not preserved.

What steps will reproduce the problem?
(1) Open a video on m.youtube.com or vimeo.com in 2D mode
(2) press the fullscreen button to start fullscreen mode
(3) Enter Chrome VR by placing device in headset

What is the expected result? Video is playing in cinema mode (fullscreen mode in Chrome VR)

What happens instead? fullscreen mode is not preserved. Video is paused, and the whole page is displayed.