Fix handling of top margin on float with shape-outside

When a float has shape outside, the top margin should be treated as if there is
no shape there, so inline content should be allowed to flow into that space.
This patch fixes two issues:

1) If the top margin is the same as the line height, a line should be able to
fit into the margin. Before this patch, that line was being treated as if it
intersected with the shape.

2) The shape should be positioned (x, y) relative to the box sizing box of the
float. While the x coordinate was being treated properly, the y coordinate was
relative to the top of the margin box. This patch fixes this behavior.

This patch also includes a simple test for right and left margins, as I wrote
that test and then discovered the problems listed above.

This patch also removes an unused override of the lineOverlapsShapeBounds
method.

BUG=260357

Review URL: https://chromiumcodereview.appspot.com/23965006

git-svn-id: svn://svn.chromium.org/blink/trunk@158060 bbb929c8-8fbe-4397-9dbb-9b2b20218538