Add assert on frameView::wasViewportResized

While going over the code, I found that renderView()->style() is used without a null check.
I'm adding an assert to clarify that we're assuming it should be there when renderView() is.

Review URL: https://codereview.chromium.org/663203003

git-svn-id: svn://svn.chromium.org/blink/trunk@184075 bbb929c8-8fbe-4397-9dbb-9b2b20218538