IDL: make optional arguments (without default) explicit sometimes

LayoutTests/fast/canvas/script-tests/canvas-isPointInPath-winding.js

 description("Series of tests to ensure correct results of the winding rule in isPointInPath.");
 
 var tmpimg = document.createElement('canvas');
 tmpimg.width = 200;
 tmpimg.height = 200;
 ctx = tmpimg.getContext('2d');
 
 // Execute test.
 function prepareTestScenario() {
     debug('Testing default isPointInPath');
@@ skipping 53 matching lines @@
     debug('Testing invalid type isPointInPath with Path object');
     shouldThrow("ctx.isPointInPath(null, 50, 50)");
     shouldThrow("ctx.isPointInPath(null, 50, 50, 'nonzero')");
     shouldThrow("ctx.isPointInPath(null, 50, 50, 'evenodd')");
     shouldThrow("ctx.isPointInPath(null, 50, 50, null)");
     shouldThrow("ctx.isPointInPath(path, 50, 50, null)");
     shouldThrow("ctx.isPointInPath(undefined, 50, 50)");
     shouldThrow("ctx.isPointInPath(undefined, 50, 50, 'nonzero')");
     shouldThrow("ctx.isPointInPath(undefined, 50, 50, 'evenodd')");
     shouldThrow("ctx.isPointInPath(undefined, 50, 50, undefined)");
-    shouldThrow("ctx.isPointInPath(path, 50, 50, undefined)");
     shouldThrow("ctx.isPointInPath([], 50, 50)");
     shouldThrow("ctx.isPointInPath([], 50, 50, 'nonzero')");
     shouldThrow("ctx.isPointInPath([], 50, 50, 'evenodd')");
     shouldThrow("ctx.isPointInPath({}, 50, 50)");
     shouldThrow("ctx.isPointInPath({}, 50, 50, 'nonzero')");
     shouldThrow("ctx.isPointInPath({}, 50, 50, 'evenodd')");
     debug('');
 }
 
 // Run test and allow variation of results.
 prepareTestScenario();