Don't allow column spanners inside transforms.
Spanners want the multicol container as their containing block. Transforms want
to be the containing block of everything inside. Since it's not possible to
fulfill both wishes, just refuse objects to become spanners when inside
transforms. We already do the same when inside out-of-flow objects, and also
for anything that establishes a new formatting context.
BUG=596863
Review URL: https://codereview.chromium.org/1908393002
Cr-Commit-Position: refs/heads/master@{#389207}
3 files changed
