Bug 1634602 - Exempt about:black from color scheme preference in RFP mode. r=emilio, a=jcristau

Differential Revision: https://phabricator.services.mozilla.com/D77833

dom/base/Document.cpp

@@ -16320,8 +16320,9 @@ void Document::RemoveToplevelLoadingDocument(Document* aDoc) {
   }
 }
-StylePrefersColorScheme Document::PrefersColorScheme() const {
-  if (nsContentUtils::ShouldResistFingerprinting(this)) {
+StylePrefersColorScheme Document::PrefersColorScheme(
+    Document::IgnoreRFP aIgnoreRFP /* = IgnoreRFP::No */) const {
+  if (aIgnoreRFP == IgnoreRFP::No && nsContentUtils::ShouldResistFingerprinting(this)) {
     return StylePrefersColorScheme::Light;
   }

dom/base/Document.h

@@ -3868,7 +3868,8 @@ class Document : public nsINode,
   nsIPermissionDelegateHandler* PermDelegateHandler();
 
   // CSS prefers-color-scheme media feature for this document.
-  StylePrefersColorScheme PrefersColorScheme() const;
+  enum class IgnoreRFP { No, Yes };
+  StylePrefersColorScheme PrefersColorScheme(IgnoreRFP aIgnoreRFP = IgnoreRFP::No) const;
 
   // Returns true if we use overlay scrollbars on the system wide or on the
   // given document.

layout/base/PresShell.cpp

@@ -5161,7 +5161,7 @@ nscolor PresShell::GetDefaultBackgroundColorToDraw() {
   BrowsingContext* bc = doc->GetBrowsingContext();
   if (bc && bc->IsTop() && !bc->HasOpener() && doc->GetDocumentURI() &&
       NS_IsAboutBlank(doc->GetDocumentURI()) &&
-      doc->PrefersColorScheme() == StylePrefersColorScheme::Dark) {
+      doc->PrefersColorScheme(Document::IgnoreRFP::Yes) == StylePrefersColorScheme::Dark) {
     // Use --in-content-page-background for prefers-color-scheme: dark.
     return NS_RGB(0x2A, 0x2A, 0x2E);
   }