[FastTextAutosizer] Do not inflate potential clusters in inflateTable

This patch changes inflateTable to not pre-inflate potential clusters.
This fixes a bug where inline block elements would appear too wide
because they were incorrectly pre-inflated in inflateTable. To
achieve this, blockMightBecomeCluster has been factored out and
is now shared by inflateTable and clusterHasEnoughTextToAutosize.

Before: http://pr.gg/wrapbefore.png
After: http://pr.gg/wrapafter.png

This patch also removes the unused function isLayoutRoot.

BUG=363893

Review URL: https://codereview.chromium.org/240443004

git-svn-id: svn://svn.chromium.org/blink/trunk@171911 bbb929c8-8fbe-4397-9dbb-9b2b20218538