Test usage of registered custom properties in custom paint

third_party/WebKit/LayoutTests/csspaint/resources/generate-paint-style-logging.js

 // Generates code for a CSS paint API function which logs the given properties
 // to the console.
 //
 // Usage:
 // generatePaintStyleLogging([
 //      '--foo',
 //      'line-height',
 // ]);
 
 function generatePaintStyleLogging(properties) {
     const json = JSON.stringify(properties);
     return `
         registerPaint('test', class {
             static get inputProperties() { return ${json}; }
             paint(ctx, geom, styleMap) {
                 const properties = styleMap.getProperties().sort();
                 for (let i = 0; i < properties.length; i++) {
                     const value = styleMap.get(properties[i]);
-                    console.log(properties[i] + ': ' + (value ? value.cssText: '[null]'));
+                    let serialized;
+                    if (value) {
+                        serialized = '[' + value.constructor.name + '=' + value.cssText + ']';
+                    } else {
+                        serialized = '[null]';
+                    }
+                    console.log(properties[i] + ': ' + serialized);
                 }
             }
         });
     `;
 }