Add "alpha" option to PaintWorklet

third_party/WebKit/LayoutTests/csspaint/registerPaint.html

Index: third_party/WebKit/LayoutTests/csspaint/registerPaint.html
diff --git a/third_party/WebKit/LayoutTests/csspaint/registerPaint.html b/third_party/WebKit/LayoutTests/csspaint/registerPaint.html
index 6dfda05e27e2866554e17a4e25ca6524baa8aa7b..e6bad4b0c03547f8c868cfc13b856a48e9a5ec24 100644
--- a/third_party/WebKit/LayoutTests/csspaint/registerPaint.html
+++ b/third_party/WebKit/LayoutTests/csspaint/registerPaint.html
@@ -59,6 +59,11 @@ function runTest() {
     }, {
         expectedMessage: "-webkit-border-radius is a prefixed property which is not supported.",
         script: "registerPaint('foo11', class { static get inputProperties() { return ['-webkit-border-radius']; } paint() { } });"
+    }, {
+        expectedError: "The 'alpha' property on the class is not a boolean.",
+        script: "registerPaint('foo12', class { static get alpha() { return 42; } });",
+    }, {
+        script: "registerPaint('foo13', class { static get alpha() { return true; } paint() { } }); console.log('Success for \\'foo13\\'.');",
     }]);
 }
 </script>