LibWeb: Add a visit_edges() override to AbstractBrowsingContext

AbstractBrowsingContext has a subclass RemoteBrowsingContext without a
visit_edges() override (and it doesn't really need one). But currently,
we rely on subclasses visiting AbstractBrowsingContext's opener BC.

This adds a visit_edges() to AbstractBrowsingContext to explicitly visit
the opener BC itself.

Meta/gn/secondary/Userland/Libraries/LibWeb/HTML/BUILD.gn

@@ -11,6 +11,7 @@ source_set("HTML") {
     "//Userland/Libraries/LibWeb:all_generated",
   ]
   sources = [
+    "AbstractBrowsingContext.cpp",
     "AnimatedBitmapDecodedImageData.cpp",
     "AttributeNames.cpp",
     "AudioTrack.cpp",