[New Multicolumn] Temporarily changing height is scary and can easily be avoided.

Storing the height and setting it to zero afterwards, just to be able to
calculate desired column height, and to determine if it's auto or not, may
have undesired side-effects, and is easily avoidable.

Review URL: https://codereview.chromium.org/239003003

git-svn-id: svn://svn.chromium.org/blink/trunk@171687 bbb929c8-8fbe-4397-9dbb-9b2b20218538