[New Multicolumn] Update sets' flow thread portion rectangles properly.

logicalHeightOfAllFlowThreadContent() was wrong. This still doesn't matter
much, but it will once we have support for multiple sets (column-span:all).

Review URL: https://codereview.chromium.org/265293005

git-svn-id: svn://svn.chromium.org/blink/trunk@173769 bbb929c8-8fbe-4397-9dbb-9b2b20218538