[New Multicolumn] Margins incorrectly accounted for before forced breaks.

[New Multicolumn] Margins incorrectly accounted for before forced breaks.

The spec basically says that margins should be ignored at column boundaries,
although it's explicitly ambiguous about what should happen to top margins
after a forced break. This CL is solely about margins before a break, though,
and they should all be set to 0, so that contents that follow start at the top
of the next column, instead of leaving one column blank and skipping to the
next one after that.

BUG=448366
R=jchaffraix@chromium.org,leviw@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=188456

[Julien - ping for review, 2015-01-15 09:43:14]

The CQ bit was checked by jchaffraix@chromium.org

[Julien - ping for review, 2015-01-15 09:43:14]

lgtm

[commit-bot: I haz the power, 2015-01-15 09:43:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/850703002/1

[commit-bot: I haz the power, 2015-01-15 09:47:20]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=188456