Introduce classifyBlock and BlockFlags enum.

These flags let us distinguish different types of (potential) clusters without repeated calls to confusingly-named blockMightBeWhatever functions.

BUG=339213

Review URL: https://codereview.chromium.org/246483002

git-svn-id: svn://svn.chromium.org/blink/trunk@172575 bbb929c8-8fbe-4397-9dbb-9b2b20218538