[New Multicolumn] Support for block direction margins on spanners.
Block direction margins need to be copied from the column-span:all element
to its placeholder, since it's the placeholder that gets laid out and
positioned among other placeholders and column sets within the multicol
container.
Also prevent margin-collapsing through column sets, since that suddenly
matters.
Note that inline direction margins were already working. Some of the
tests in this CL also test those.
BUG=347325
R=jchaffraix@chromium.org
Review URL: https://codereview.chromium.org/855413002
git-svn-id: svn://svn.chromium.org/blink/trunk@188696 bbb929c8-8fbe-4397-9dbb-9b2b20218538
26 files changed
