Handle NULL frame in HTMLFrameOwnerElement::UpdateRequiredPolicy()

Previously it is assumed when a frame owner element needs to update
required policy, |GetDocument().GetFrame()| should not return NULL.
It turns out this assumption is not always true.

This CL let HTMLFrameOwnerElement::UpdateRequiredPolicy handle
NULL frame explicitly, instead of assuming its not NULL.

Bug: 1068100
Change-Id: I15e6ac7cffeff36761da945b67eca3f862719942
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2149527
Commit-Queue: Charlie Hu <chenleihu@google.com>
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760077}

third_party/blink/renderer/core/html/html_frame_owner_element.cc

@@ -322,11 +322,12 @@ void HTMLFrameOwnerElement::UpdateContainerPolicy(Vector<String>* messages) {
 
 void HTMLFrameOwnerElement::UpdateRequiredPolicy() {
   const auto* frame = GetDocument().GetFrame();
-  DCHECK(frame);
   DocumentPolicy::FeatureState new_required_policy =
-      DocumentPolicy::MergeFeatureState(
-          ConstructRequiredPolicy(), /* self_required_policy */
-          frame->GetRequiredDocumentPolicy() /* parent_required_policy */);
+      frame
+          ? DocumentPolicy::MergeFeatureState(
+                ConstructRequiredPolicy(), /* self_required_policy */
+                frame->GetRequiredDocumentPolicy() /* parent_required_policy */)
+          : ConstructRequiredPolicy();
 
   // Filter out policies that are disabled by origin trials.
   frame_policy_.required_document_policy.clear();