Squashed fixed position layers shouldn't flicker while scrolling

We were correctly squashing fixed position layers together, but we weren't
setting the correct position constraint on the squashing layer. Instead, we
were setting the constraint on the main graphics layer directly, which meant
the compositor incorrectly scrolled the squashed fixed position layer instead
of keeping it stuck to the viewport.

R=vollick@chromium.org
BUG=402819

Review URL: https://codereview.chromium.org/475553002

git-svn-id: svn://svn.chromium.org/blink/trunk@180225 bbb929c8-8fbe-4397-9dbb-9b2b20218538