Fix crash with fullscreen elements inside frames.

My patch https://src.chromium.org/viewvc/blink?view=rev&revision=183623
wrongly assumed that the fullscreen element was on the main frame.  Use
the frame tracked by FullscreenController instead.

NOTRY=true
BUG=423457

Review URL: https://codereview.chromium.org/647343003

git-svn-id: svn://svn.chromium.org/blink/trunk@183711 bbb929c8-8fbe-4397-9dbb-9b2b20218538