IDL: make optional arguments (without default) explicit sometimes

LayoutTests/fast/canvas/script-tests/canvas-path-context-fill.js

Index: LayoutTests/fast/canvas/script-tests/canvas-path-context-fill.js
diff --git a/LayoutTests/fast/canvas/script-tests/canvas-path-context-fill.js b/LayoutTests/fast/canvas/script-tests/canvas-path-context-fill.js
index 71b1f849e422c8f61835779a5010c58649c72d38..3bf0d09938e49f9014d6f3bed9fba10bd27f59e2 100644
--- a/LayoutTests/fast/canvas/script-tests/canvas-path-context-fill.js
+++ b/LayoutTests/fast/canvas/script-tests/canvas-path-context-fill.js
@@ -73,10 +73,8 @@ function prepareTestScenario() {
     shouldThrow("ctx.fill({}, 'evenodd')");
     shouldThrow("ctx.fill('gazonk')");
     shouldThrow("ctx.fill(path, 'gazonk')");
-    shouldThrow("ctx.fill(undefined)");
     shouldThrow("ctx.fill(undefined, undefined)");
     shouldThrow("ctx.fill(undefined, path)");
-    shouldThrow("ctx.fill(path, undefined)");
 }
 
 // Run test and allow variation of results.