(Re-)introduce AncestorThrottle to handle 'X-Frame-Options'.

content/browser/loader/navigation_resource_throttle.cc

Index: content/browser/loader/navigation_resource_throttle.cc
diff --git a/content/browser/loader/navigation_resource_throttle.cc b/content/browser/loader/navigation_resource_throttle.cc
index 7a725a8e85ff056e16c4b0a71e8eddf6e05c3532..5ce1654f7660b3a0cd17866eef12273dee66d62c 100644
--- a/content/browser/loader/navigation_resource_throttle.cc
+++ b/content/browser/loader/navigation_resource_throttle.cc
@@ -345,6 +345,16 @@ void NavigationResourceThrottle::OnUIChecksPerformed(
     Cancel();
   } else if (result == NavigationThrottle::BLOCK_REQUEST) {
     CancelWithError(net::ERR_BLOCKED_BY_CLIENT);
+  } else if (result == NavigationThrottle::BLOCK_RESPONSE) {
+    // TODO(mkwst): If we cancel the main frame request with anything other than
+    // 'net::ERR_ABORTED', we'll trigger some special behavior that might not be
+    // desirable here (non-POSTs will reload the page, while POST has some logic
+    // around reloading to avoid duplicating actions server-side). For the
+    // moment, only child frame navigations should be blocked. If we need to
+    // block main frame navigations in the future, we'll need to carefully
+    // consider the right thing to do here.
+    DCHECK(!ResourceRequestInfo::ForRequest(request_)->IsMainFrame());
+    CancelWithError(net::ERR_BLOCKED_BY_RESPONSE);
   } else {
     Resume();
   }