Add "alpha" option to PaintWorklet

third_party/WebKit/Source/modules/csspaint/PaintWorkletGlobalScope.cpp

Index: third_party/WebKit/Source/modules/csspaint/PaintWorkletGlobalScope.cpp
diff --git a/third_party/WebKit/Source/modules/csspaint/PaintWorkletGlobalScope.cpp b/third_party/WebKit/Source/modules/csspaint/PaintWorkletGlobalScope.cpp
index 1f33dae2b993faefe2992108a8c4627844348b23..08e4744b507e1222e73784321e8610dca7452d31 100644
--- a/third_party/WebKit/Source/modules/csspaint/PaintWorkletGlobalScope.cpp
+++ b/third_party/WebKit/Source/modules/csspaint/PaintWorkletGlobalScope.cpp
@@ -89,6 +89,16 @@ void PaintWorkletGlobalScope::registerPaint(const String& name, const ScriptValu
         }
     }
 
+    // Parse 'alpha' AKA hasAlpha property.
+    v8::Local<v8::Value> alphaValue;
+    if (!v8Call(constructor->Get(context, v8String(isolate, "alpha")), alphaValue))
+        return;
+    if (!isUndefinedOrNull(alphaValue) && !alphaValue->IsBoolean()) {
+        exceptionState.throwTypeError("The 'alpha' property on the class is not a boolean.");
+        return;
+    }
+    bool hasAlpha = v8::Local<v8::Boolean>::Cast(alphaValue)->Value();
+
     v8::Local<v8::Value> prototypeValue;
     if (!v8Call(constructor->Get(context, v8String(isolate, "prototype")), prototypeValue))
         return;
@@ -121,7 +131,7 @@ void PaintWorkletGlobalScope::registerPaint(const String& name, const ScriptValu
 
     v8::Local<v8::Function> paint = v8::Local<v8::Function>::Cast(paintValue);
 
-    CSSPaintDefinition* definition = CSSPaintDefinition::create(scriptController()->getScriptState(), constructor, paint, nativeInvalidationProperties, customInvalidationProperties);
+    CSSPaintDefinition* definition = CSSPaintDefinition::create(scriptController()->getScriptState(), constructor, paint, nativeInvalidationProperties, customInvalidationProperties, hasAlpha);
     m_paintDefinitions.set(name, definition);
 
     // Set the definition on any pending generators.