[CSS Paint API] Add StylePropertyMap as an argument to the paint function.

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.cssString : '[null]'));
+                }
+            }
+        });
+    `;
+}