Simply rules for compositing fixed position elements

Previously we had many complex conditions for determining whether a fixed
position element ought to be composited. This CL simplifies those conditions to
just checking whether the fixed position element's container is actually the
viewport.

Review URL: https://codereview.chromium.org/449723003

git-svn-id: svn://svn.chromium.org/blink/trunk@179749 bbb929c8-8fbe-4397-9dbb-9b2b20218538