New positioning model: Borders

Converting to the shape coordinate system was not properly accounting
for borders and padding. This has been fixed and a new convienence
method added because of the added complexity of the conversion.

This is a port from WebKit: http://trac.webkit.org/changeset/153058

BUG=260357

Review URL: https://chromiumcodereview.appspot.com/20107003

git-svn-id: svn://svn.chromium.org/blink/trunk@154887 bbb929c8-8fbe-4397-9dbb-9b2b20218538