Prepare RenderLayer::boundingBox for unification with calculateLayerBounds

RenderLayer has multiple code paths for calculating the bounds of the
RenderLayer, each of which understands some subset of the features in the
engine. This CL prepares us to unify RenderLayer::boundingBox and
calculateLayerBounds by simplifying boundingBox and renaming it to
physicalBoundingBox.

Eventually, this change will let us save 7% of the compositing update in the
calculator sample app because the overlap map and the CompositedLayerMapping
will be able to use the same bounds calculation.

Unfortunately, this CL removes support for accounting for fragments in the
RenderLayer's bounding box. Previously, accounting for fragments was optional,
which meant that a RenderLayer didn't have a clearly defined
physicalBoundingBox. Removing this functionality appears to break Region-based
multicolumn, which isn't a shipping feature. The underlying problem is that
Region-based multicolumn doesn't integrate correctly with compositing (e.g.,
fragments weren't accounted for in the overlap test). We should re-introduce
support for fragments once we've unified the various bounding box computations.
Hopefully the end result will be that Region-based multicolumn will be easier
to integrate properly with compositing.

Review URL: https://codereview.chromium.org/224833003

git-svn-id: svn://svn.chromium.org/blink/trunk@170835 bbb929c8-8fbe-4397-9dbb-9b2b20218538