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

Added support for the CSS shape-image-threshold property. This change just enables parsing the property, it's not connected to the Shapes code yet. The number valued shape-image-threshold property is defined here: http://dev.w3.org/csswg/css-shapes-1/#shape-image-threshold-property.

Its computed value is clamped to the range [0, 1].

BUG=303198

Review URL: https://codereview.chromium.org/25660008

git-svn-id: svn://svn.chromium.org/blink/trunk@158939 bbb929c8-8fbe-4397-9dbb-9b2b20218538