Fix WPT for Referrer Policy about:blank inheritance

When Referrer-Policy is `origin`, we should expect the URL in the
Referer header to be document's origin with a trailing '/', see
https://w3c.github.io/webappsec-referrer-policy/#referrer-policy-origin

Change-Id: I720dbf0892701b380aebab8d7b8c4aa55098c88c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438423Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Antonio Sartori <antoniosartori@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812621}

third_party/blink/web_tests/external/wpt/referrer-policy/generic/inheritance/iframe-inheritance-about-blank-expected.txt

 This is a testharness.js-based test.
 PASS The fetch() API in an about:blank iframe with the 'client' referrer is fetched with no 'Referer' header
-FAIL The fetch() API in an about:blank iframe with a custom URL referrer is fetched with a 'Referer` header that uses the outer document's URL along with its referrer policy assert_equals: expected "http://web-platform.test:8001" but got "http://web-platform.test:8001/referrer-policy/generic/inheritance/iframe-inheritance-about-blank.html/custom"
+FAIL The fetch() API in an about:blank iframe with a custom URL referrer is fetched with a 'Referer` header that uses the outer document's URL along with its referrer policy assert_equals: expected "http://web-platform.test:8001/" but got "http://web-platform.test:8001/referrer-policy/generic/inheritance/iframe-inheritance-about-blank.html/custom"
 FAIL The value of document.referrer in an about:blank iframe is the outer document's full URL, regardless of referrer policy assert_equals: expected "http://web-platform.test:8001/referrer-policy/generic/inheritance/iframe-inheritance-about-blank.html" but got "http://web-platform.test:8001/"
 PASS A subresource fetched from an about:blank iframe is fetched with no 'Referer' header
 Harness: the test ran to completion.

third_party/blink/web_tests/external/wpt/referrer-policy/generic/inheritance/iframe-inheritance-about-blank.html

@@ -42,7 +42,7 @@ window.addEventListener("message", msg => {
     // inherits its parent's referrer policy, the URL should be restricted to
     // its origin.
     testFetchURLReferrer.step_func_done(() => {
-      assert_equals(referrer, location.origin);
+      assert_equals(referrer, location.origin + '/');
     })();
   } else if (test_name === "testDocumentReferrer") {
     // The referrer of the initial document in an about:blank iframe is set to