[CSS Shapes] Empty polygons with non-zero shape-padding cause an ASSERT crash

PolygonShape::shapePaddingBounds() and PolygonShape::shapeMarginBounds() no longer attempt to compute a new FloatPolygon when the original is empty, i.e. when it has less than three vertices.

This patch fixes the clusterfuzz failure reported here: https://code.google.com/p/chromium/issues/detail?id=314402

BUG=319069, 314402

Review URL: https://codereview.chromium.org/68263018

git-svn-id: svn://svn.chromium.org/blink/trunk@161996 bbb929c8-8fbe-4397-9dbb-9b2b20218538