RenderBlockFlow::nextFloatLogicalBottomBelow should not use ShapeOutsideFloatOffsetMode

Rewrite nextFloatLogicalBottomBelow to use the placed floats tree for the
search and to not need ShapeOutsideFloatOffsetMode anymore. This moves almost
all of the logic into FloatingObjects, making a small reduction in the amount
that RenderBlockFlow needs to know about the implementation of FloatingObjects.

In addition, change ComputeFloatOffsetAdapter to take in LayoutUnits and
roundToInt itself so that all of it's callers can be simplified.

Review URL: https://codereview.chromium.org/68123004

git-svn-id: svn://svn.chromium.org/blink/trunk@162056 bbb929c8-8fbe-4397-9dbb-9b2b20218538