[New Multicolumn] boundingBoxForCompositingOverlapTest() needs to use fragmentsBoundingBox().

Bounding boxes are visual, so we need to translate a flow thread
rectangle to the union of each visual fragment in each column that the
layer occurs. This means that flippedLogicalBoundingBox() won't do. Nor
will physicalBoundingBox(), which, despite its name, isn't "physical"
enough for multicol. In our case, flippedLogicalBoundingBox() and
physicalBoundingBox() would behave identically anyway, since the
ancestor layer is the same as this layer (and physicalBoundingBox()
only adds the offset from this layer to the ancestor).

BUG=359877
R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/655003005

git-svn-id: svn://svn.chromium.org/blink/trunk@184670 bbb929c8-8fbe-4397-9dbb-9b2b20218538