Fix crash when resizing fullscreened document elements.

https://codereview.chromium.org/636363003/ assumed that a
RenderFullScreen object is always created when we go fullscreen.
Actually, Fullscreen::didEnterFullScreenForElement specifically does not
create one when the element being fullscreened is the document itself.
These don't need to be manually resized either since they have a default
fit-to-viewport style, so simply early-return in that case.

NOTRY=true
BUG=425135

Review URL: https://codereview.chromium.org/671623002

git-svn-id: svn://svn.chromium.org/blink/trunk@184036 bbb929c8-8fbe-4397-9dbb-9b2b20218538