Per spec, float positioning and stacking is not affected by defining a float
area with a shape.
https://drafts.csswg.org/css-shapes/#relation-to-box-model-and-float-behavior
So all the call sites of GetFloatAvailableSpace() related to adding a
float are replaced by GetFloatAvailableSpaceForPlacingFloat().
<shape-box> with border-radius will be implemented in next part.
MozReview-Commit-ID: 1RXEeXDhdWo
