Make column snapping optional when translating to flow thread coordinates.

positionForPoint() wants this, but mapAncestorToLocal() requires that no
special behavior be applied.

While this CL doesn't fix bug 663062, it is a prerequisite for fixing it
without breaking existing tests (MulticolWithAbsPosNotContained in
MapCoordinatesTest unit test).

BUG=663062

Review-Url: https://codereview.chromium.org/2590463002
Cr-Commit-Position: refs/heads/master@{#439758}
4 files changed