[New Multicolumn] Faster and more direct RenderMultiColumnSet::multiColumnFlowThread().

No need to go via the parent block to get the flow thread.

Review URL: https://codereview.chromium.org/254883003

git-svn-id: svn://svn.chromium.org/blink/trunk@173025 bbb929c8-8fbe-4397-9dbb-9b2b20218538