page.waitFor should send ...args to page.waitForFunction (#775)

This lets the user pass `...args` into `page.waitFor`. It also clarifies that the docs that `options` is not optional if `...args` are specified.

Fixes #770

test/test.js

@@ -530,6 +530,9 @@ describe('Page', function() {
       await page.waitFor({foo: 'bar'}).catch(e => error = e);
       expect(error.message).toContain('Unsupported target type');
     }));
+    it('should wait for predicate with arguments', SX(async function() {
+      await page.waitFor((arg1, arg2) => arg1 !== arg2, {}, 1, 2);
+    }));
   });
 
   describe('Page.Events.Console', function() {