IDL: make optional arguments (without default) explicit sometimes

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

Index: LayoutTests/fast/canvas/script-tests/canvas-path-context-clip.js
diff --git a/LayoutTests/fast/canvas/script-tests/canvas-path-context-clip.js b/LayoutTests/fast/canvas/script-tests/canvas-path-context-clip.js
index 8d6a1fc4e47156c084515b61a82c0ec8f975d85c..9135da34307db990c6c572537a0b9243cb5fb9da 100644
--- a/LayoutTests/fast/canvas/script-tests/canvas-path-context-clip.js
+++ b/LayoutTests/fast/canvas/script-tests/canvas-path-context-clip.js
@@ -75,10 +75,8 @@ function prepareTestScenario() {
     shouldThrow("ctx.clip({}, 'evenodd')");
     shouldThrow("ctx.clip('gazonk')");
     shouldThrow("ctx.clip(path, 'gazonk')");
-    shouldThrow("ctx.clip(undefined)");
     shouldThrow("ctx.clip(undefined, undefined)");
     shouldThrow("ctx.clip(undefined, 'nonzero')");
-    shouldThrow("ctx.clip(path, undefined)");
 }
 
 // Run test and allow variation of results.