Make cluster creation independent of fingerprinting.  Keep track of current
cluster during layout without walking up the tree.

BUG=302005

Review URL: https://codereview.chromium.org/101543009

git-svn-id: svn://svn.chromium.org/blink/trunk@164800 bbb929c8-8fbe-4397-9dbb-9b2b20218538