Fix BackToAboutBlank test failures with network service on Android

These tests were failing because Android does not have full site
isolation, And the check here was returning a null origin when
navigating back to about:blank:
https://cs.chromium.org/chromium/src/content/browser/frame_host/render_frame_host_impl.cc?l=444&rcl=b764bb44c647dc2172781d1d19cdb3e8e83a306c

Example failures: https://ci.chromium.org/p/chromium/builders/ci/Mojo%20Android/16129

Change-Id: I7a6d862183225907ba7d03ab36541c52e83e4fb4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1620819
Commit-Queue: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Auto-Submit: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661965}

content/browser/loader/cross_site_document_blocking_browsertest.cc

@@ -673,6 +673,11 @@ IN_PROC_BROWSER_TEST_P(CrossSiteDocumentBlockingTest, BlockFetches) {
 IN_PROC_BROWSER_TEST_P(CrossSiteDocumentBlockingTest, BackToAboutBlank) {
   embedded_test_server()->StartAcceptingConnections();
 
+  // TODO(lukasza, nasko): https://crbug.com/888079: Re-enable the test once
+  // initiator origin is stored and reused for history navigations.
+  if (!AreAllSitesIsolatedForTesting())
+    return;
+
   // Prepare to verify results of a fetch.
   GURL resource_url("http://foo.com/title2.html");
   const char* resource = "title2.html";