position:sticky should stick for the enclosing overflow ancestor

Make position: -webkit-sticky be constrained by an enclosing ancestor
with non-visible overflow if there is one, rather than being constained
always by the viewport.

Port of changeset from webkit: http://trac.webkit.org/changeset/150395

BUG=322972, 231753

Review URL: https://codereview.chromium.org/103213002

git-svn-id: svn://svn.chromium.org/blink/trunk@164812 bbb929c8-8fbe-4397-9dbb-9b2b20218538