[Document Policy] Move ForceLoadAtTop check after document policy initialization

The fix to missing document policy in XSLT documents involves initialize
document policy from previous security context, which results
|DocumentLoader::document_policy_| not having correct policy content
before document policy initialization in |SecurityContextInit|.

This CL moves the check for ForceLoadAtTop feature
after document policy initialization, so that its behaviour is
consistent in XSLT documents.

Bug: 1151954
Change-Id: I0a4061975a26cbf922b322c908ce8c2c88897715
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2568439Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Commit-Queue: Charlie Hu <chenleihu@google.com>
Cr-Commit-Position: refs/heads/master@{#833070}

third_party/blink/renderer/core/loader/document_loader.cc

@@ -1679,20 +1679,6 @@ void DocumentLoader::CommitNavigation() {
       owner_document = owner_local_frame->GetDocument();
   }
 
-  // Re-validate Document Policy feature before installing the new document.
-  if (!RuntimeEnabledFeatures::DocumentPolicyEnabled(
-          owner_document ? owner_document->GetExecutionContext() : nullptr)) {
-    document_policy_ = DocumentPolicy::ParsedDocumentPolicy{};
-  }
-
-  if (document_policy_.feature_state.contains(
-          mojom::blink::DocumentPolicyFeature::kForceLoadAtTop)) {
-    navigation_scroll_allowed_ = !(
-        document_policy_
-            .feature_state[mojom::blink::DocumentPolicyFeature::kForceLoadAtTop]
-            .BoolValue());
-  }
-
   LocalDOMWindow* previous_window = frame_->DomWindow();
   InitializeWindow(owner_document);
 
@@ -1726,6 +1712,9 @@ void DocumentLoader::CommitNavigation() {
       document_policy_,
       response_.HttpHeaderField(http_names::kDocumentPolicyReportOnly));
 
+  navigation_scroll_allowed_ = !frame_->DomWindow()->IsFeatureEnabled(
+      mojom::blink::DocumentPolicyFeature::kForceLoadAtTop);
+
   WillCommitNavigation();
 
   Document* document = frame_->DomWindow()->InstallNewDocument(