Multi-column properties in a flexbox hide its content

In a flexbox if you set any of the multi-column properties (column-*)
the content of the flexbox is hidden.

According to the spec http://www.w3.org/TR/css3-flexbox/#propdef-flex
muti-column properties should be ignored:

"all of the column-* properties in the Multicol module have no
effect on a flex container."

This patch fixes this bug by adding a check in RenderBlock::requiresColumns
that columns are not required for Flexible box.

BUG=402124

Review URL: https://codereview.chromium.org/488183003

git-svn-id: svn://svn.chromium.org/blink/trunk@180776 bbb929c8-8fbe-4397-9dbb-9b2b20218538