Make ClientHintsPreferences class work like a class.

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index 34be6aec17895c23e5923f6479206e51e4d1d4a4..1a4f2e01aa4ad9f9b71e1ebd7aed5bcb0e31a2a1 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -1022,8 +1022,7 @@ public:
 
     bool isPrivilegedContext(String& errorMessage, const PrivilegeContextCheck = StandardPrivilegeCheck) const override;
 
-    void setClientHintsPreferences(const ClientHintsPreferences& preferences) { m_clientHintsPreferences.set(preferences); }
-    const ClientHintsPreferences& clientHintsPreferences() const { return m_clientHintsPreferences; }
+    ClientHintsPreferences& clientHintsPreferences() { return m_clientHintsPreferences; }
 
     CanvasFontCache* canvasFontCache();