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Only block containers that don't establish new formatting contexts are
allowed to ignore (and thus potentially overlap with) floats.

This obviously also applies inside multicol, but we had a check that actively
prevented that. This was some residue from CSS regions support (shared
implementation with multicol), which had a much more sophisticated positioning
implementation, which we've removed ages ago.

BUG=528179
R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/1325323002

git-svn-id: svn://svn.chromium.org/blink/trunk@201867 bbb929c8-8fbe-4397-9dbb-9b2b20218538