Simplify conditions in Document::updateLayout

If the document is active, we necessarily have a FrameView and a RenderView.

Also, when we're done with layout, we can just advance to LayoutClean
unconditionally.

Review URL: https://codereview.chromium.org/242613003

git-svn-id: svn://svn.chromium.org/blink/trunk@171956 bbb929c8-8fbe-4397-9dbb-9b2b20218538