[CSS Shapes] Use unsigned for the internal representation of shape-margin for image valued shapes

RasterShape now converts shape-margin to an unsigned internally. Additionally, it now uses ceil() to convert the floating point (Length) CSS property to its internal value, for the sake of consistency with the WebKit version. I expect that the Shapes spec will eventually make this detail a requirement (http://dev.w3.org/csswg/css-shapes-1/#shape-margin-property).

No new tests were included since this is just an internal clean-up.


BUG=304256

Review URL: https://codereview.chromium.org/26282004

git-svn-id: svn://svn.chromium.org/blink/trunk@159285 bbb929c8-8fbe-4397-9dbb-9b2b20218538