[New Multicolumn] Use the return value from flipForWritingMode().

flipForWritingMode() comes in at least two flavors; one that takes a
LayoutRect& and one that takes a LayoutSize&. The one that takes
LayoutRect puts the result in the reference passed (and returns nothing).
The one that takes LayoutSize leaves the reference passed untouched
(after all, it's const) and returns the result. Not confusing at all.

Better not ignore the returned value when using the LayoutSize version.

This fixes fast/multicol/vertical-rl/composited-relpos-overlapping-will-change.html

BUG=359877
R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/696523003

git-svn-id: svn://svn.chromium.org/blink/trunk@184757 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Begin migrating InlineBox to LayoutUnits.

Starting out with a relatively small patch to
make sure this approach is going to work.

BUG=321237

Review URL: https://codereview.chromium.org/690293002

git-svn-id: svn://svn.chromium.org/blink/trunk@184758 bbb929c8-8fbe-4397-9dbb-9b2b20218538