Fix sporadic crashes in TextAutosizer::processSubtree

cluster can sometimes be null when we are trying to autosize an orhpaned
subtree of the document; in this case should abort.

TBR=pdr@chromium.org
NOTRY=true
BUG=none

Review URL: https://codereview.chromium.org/112093002

git-svn-id: svn://svn.chromium.org/blink/trunk@163571 bbb929c8-8fbe-4397-9dbb-9b2b20218538