Fix supercluster logic.

The cluster stack now owns the Cluster objects; we no longer "pre-create" a Cluster before its root has entered layout.  Instead we connect it to a Supercluster object which holds the set of roots that share its fingerprint, and the common multiplier.

The multiplier for the supercluster is based on the deepest common ancestor of the deepest-block-containing-all-text for all its clusters.

BUG=302005

Review URL: https://codereview.chromium.org/145123010

git-svn-id: svn://svn.chromium.org/blink/trunk@167133 bbb929c8-8fbe-4397-9dbb-9b2b20218538