Fix incorrect unreachable_func in CSP layout tests.

Credit goes to nick@ for spotting it in
https://chromium-review.googlesource.com/c/chromium/src/+/999182

Change-Id: I278234762e4ca8ced592c0b63636c6dc6b981e6f
Reviewed-on: https://chromium-review.googlesource.com/1010473Reviewed-by: Nick Carter <nick@chromium.org>
Commit-Queue: Nick Carter <nick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552077}

third_party/WebKit/LayoutTests/external/wpt/content-security-policy/embedded-enforcement/support/testharness-helper.sub.js

@@ -118,7 +118,7 @@ function assert_iframe_with_csp(t, url, csp, shouldBlock, urlId, blockedURI) {
     window.onmessage = function (e) {
       if (e.source != i.contentWindow)
           return;
-      t.unreached_func('No message should be sent from the frame.');
+      t.assert_unreached('No message should be sent from the frame.');
     }
     i.onload = t.step_func(function () {
       // Delay the check until after the postMessage has a chance to execute.