[CSS Shapes] Positioning floats inside shape-inside should respect the shape's boundaries

We recompute the shape's segments for the line based on the float's height, so it's going to be
positioned correctly inside a shape-inside. This change is just a workaround for the problem, the
proper and nice fix is coming later.

R=eric@chromium.org
BUG=273100

Review URL: https://chromiumcodereview.appspot.com/22996004

git-svn-id: svn://svn.chromium.org/blink/trunk@156120 bbb929c8-8fbe-4397-9dbb-9b2b20218538