Use the outermost float for shape-outside

Shape outside was using the last float encountered on the line, assuming it
would always be the outermost float. However, this is not necessarily true, so
switch to using the outermost float. This has the advantage of also only
needing one saved float when computing the offset, not two.

BUG=271774

Review URL: https://chromiumcodereview.appspot.com/23084002

git-svn-id: svn://svn.chromium.org/blink/trunk@156038 bbb929c8-8fbe-4397-9dbb-9b2b20218538