[CSS Shapes] Support the shape-image-threshold property

Replace the hardwired 0 Shape::createShape() threshold parameter with the style's shapeImageThreshold() value (which has been clamped to [0,1]).

BUG=303453

Review URL: https://codereview.chromium.org/25968007

git-svn-id: svn://svn.chromium.org/blink/trunk@158953 bbb929c8-8fbe-4397-9dbb-9b2b20218538