IDL: make optional arguments (without default) explicit sometimes

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

 description("Series of tests to ensure clip() works with path and winding rule parameters.");
 
 var ctx = document.getElementById('canvas').getContext('2d');
 
 function pixelDataAtPoint() {
   return ctx.getImageData(50, 50, 1, 1).data;
 }
 
 function checkResult(expectedColors, sigma) {
     for (var i = 0; i < 4; i++)
@@ skipping 57 matching lines @@
     shouldThrow("ctx.clip(null, null)");
     shouldThrow("ctx.clip(null, 'nonzero')");
     shouldThrow("ctx.clip(path, null)");
     shouldThrow("ctx.clip([], 'nonzero')");
     shouldThrow("ctx.clip({}, 'nonzero')");
     shouldThrow("ctx.clip(null, 'evenodd')");
     shouldThrow("ctx.clip([], 'evenodd')");
     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.
 prepareTestScenario();