Enable CrossSiteDocumentBlockingTest.BackToAboutBlank on Android.

Since r708618 we are using the initiator origin (rather than process
lock fallback) for calculating request_initiator_site_lock.  Because
of this, it should be fine to enable BackToAboutBlank test even if
we don't have a guarantee that all processes are locked.

Bug: 888079
Change-Id: Ied1feaaff41dbc693416c9072f2f3d0849c7aabc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031930Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737131}

content/browser/loader/cross_site_document_blocking_browsertest.cc

@@ -717,11 +717,6 @@ IN_PROC_BROWSER_TEST_P(CrossSiteDocumentBlockingTest,
 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";