[CSS Shapes] Turn Shape's logicalwidth/height into a LayoutSize

In order to keep things compact and match with WebKit, I modified shape's implementation to use
LayoutSize rather than 2 LayoutUnits to store the Shapes's dimension.

Merged WebKit revision: http://trac.webkit.org/changeset/155626

Review URL: https://chromiumcodereview.appspot.com/23710058

git-svn-id: svn://svn.chromium.org/blink/trunk@157856 bbb929c8-8fbe-4397-9dbb-9b2b20218538