Use context document's ResourceFetcher in FrameSerializer

This is a follow up change for
https://crrev.com/8ee3f7e6d3a79b15824b6e286eb71c0a96c4d879. See its CL
description for details.

Bug: 961614, 1006292
Change-Id: Idff6f6645cf01cc7ea8c873cbde9039e641f7ecc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1862014Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706289}

third_party/blink/renderer/core/frame/frame_serializer.cc

@@ -30,6 +30,8 @@
 
 #include "third_party/blink/renderer/core/frame/frame_serializer.h"
 
+#include "base/feature_list.h"
+#include "third_party/blink/public/common/features.h"
 #include "third_party/blink/renderer/core/css/css_font_face_rule.h"
 #include "third_party/blink/renderer/core/css/css_font_face_src_value.h"
 #include "third_party/blink/renderer/core/css/css_image_value.h"
@@ -630,7 +632,19 @@ void FrameSerializer::RetrieveResourcesForCSSValue(const CSSValue& css_value,
     if (font_face_src_value->IsLocal())
       return;
 
-    AddFontToResources(font_face_src_value->Fetch(&document, nullptr));
+    if (base::FeatureList::IsEnabled(
+            features::kHtmlImportsRequestInitiatorLock)) {
+      if (Document* context_document = document.ContextDocument()) {
+        // For @imports from HTML imported Documents, we use the
+        // context document for getting origin and ResourceFetcher to use the
+        // main Document's origin, while using the element document for
+        // CompleteURL() to use imported Documents' base URLs.
+        AddFontToResources(
+            font_face_src_value->Fetch(context_document, nullptr));
+      }
+    } else {
+      AddFontToResources(font_face_src_value->Fetch(&document, nullptr));
+    }
   } else if (const auto* css_value_list = DynamicTo<CSSValueList>(css_value)) {
     for (unsigned i = 0; i < css_value_list->length(); i++)
       RetrieveResourcesForCSSValue(css_value_list->Item(i), document);