[New Multicolumn] Make layer bounding boxes visual and relative to "root".

The bounding box needs to be the union of all the box's fragments.
There's one fragment for each column in which the box occurs.

In addition to being a union of all fragments, the bounding box needs to
be relative to the "root"/"ancestor" layer. If the root layer is also
inside the fragmentation context, we need to find the visual position of
the root layer as well (which may very well be in a different column than
the layer we're working on).

A lot was already almost correct in RenderLayer::fragmentsBoundingBox(),
while it was completely missing in RenderLayer::boundingBoxForCompositing().

Added a helper convertFromFlowThreadToVisualBoundingBoxInAncestor() to
deal with this piece of advanced technology in one place.

BUG=359877

Review URL: https://codereview.chromium.org/645023002

git-svn-id: svn://svn.chromium.org/blink/trunk@183766 bbb929c8-8fbe-4397-9dbb-9b2b20218538