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We were missing the case where the first object in a multicol container was a
spanner (the call to previousSiblingMultiColumnSet() should have been
previousSiblingMultiColumnBox(), to catch spanner placeholders in addition to
column sets).

But instead of having a special code path depending on whether we're dealing
with the first box or not (to avoid subtracting the multicol container's top
border and padding from an uncalculated logical top of a column set), always
subtract the margin top edge of the first column box instead.

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/1631633002

Cr-Commit-Position: refs/heads/master@{#371661}