[CSS Shapes] Basic shapes' computed position should be a horizontal and vertical offset

This patch updates the computed position values used for blending to be a horizontal
left offset and a vertical top offset. When positions include a center, bottom, or
right position offset, it is converted to the appropriate top/left coordinate as
a calc expression. Serialized values still use the original bottom/right directions
when present, and also omit the top/left keywords where possible.

A test is added to shape-outside.html since it had no test for interpolation of extended positions.

Patch by Bear Travis.

BUG=308251

Review URL: https://codereview.chromium.org/144373002

git-svn-id: svn://svn.chromium.org/blink/trunk@168180 bbb929c8-8fbe-4397-9dbb-9b2b20218538