Fix shape-outside on floats with padding

When the top of the line was converted to shape coordinates, the padding
was being added in an extra time, causing shapes to be improperly
placed. This fixes that, and adds in a whole host of tests for
shape-outside on floats with padding.

It also fixes a few tests that were inadvertantly relying on the old
behavior. These tests were also changed to be simpler and less fragile
in the future.

BUG=260357

Review URL: https://codereview.chromium.org/47073011

git-svn-id: svn://svn.chromium.org/blink/trunk@161277 bbb929c8-8fbe-4397-9dbb-9b2b20218538