[WPT/common/security-features] Stop using assert_* from common.js

As preparation for
https://chromium-review.googlesource.com/c/chromium/src/+/1565621
this CL replaces assert_*() with Promise.reject() or throw,
to enable common.js to work without testharness.js and outside
promise_test().

Bug: 906850
Change-Id: Iea2b4621c44c0c28aaf1f9bb0a1412e4ef682c4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1565556
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653508}

third_party/blink/web_tests/external/wpt/common/security-features/resources/common.js

@@ -263,7 +263,8 @@ function requestViaIframe(url, additionalAttributes) {
       false);
   return bindEvents2(window, "message", iframe, "error", window, "error")
       .then(event => {
-          assert_equals(event.source, iframe.contentWindow);
+          if (event.source !== iframe.contentWindow)
+            return Promise.reject(new Error('Unexpected event.source'));
           return event.data;
         });
 }
@@ -481,8 +482,7 @@ function get_worklet(type) {
   if (type == 'audio')
     return new OfflineAudioContext(2,44100*40,44100).audioWorklet;
 
-  assert_unreached('unknown worklet type is passed.');
-  return undefined;
+  throw new Error('unknown worklet type is passed.');
 }
 
 function requestViaWorklet(type, url) {
@@ -510,7 +510,8 @@ function requestViaNavigable(navigableElement, url) {
   const promise =
     bindEvents2(window, "message", iframe, "error", window, "error")
       .then(event => {
-          assert_equals(event.source, iframe.contentWindow, "event.source");
+          if (event.source !== iframe.contentWindow)
+            return Promise.reject(new Error('Unexpected event.source'));
           return event.data;
         });
   navigableElement.click();