Remove the mediaFullscreenRequiresUserGesture setting

It is no longer set by Chromium and is always true.

BUG=355439

Review URL: https://codereview.chromium.org/212323003

git-svn-id: svn://svn.chromium.org/blink/trunk@170416 bbb929c8-8fbe-4397-9dbb-9b2b20218538