LibWeb: Add Document::is_fully_active

This is used in a bunch of places in the HTML spec. The current use
case for this is History.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -970,4 +970,12 @@ String Document::referrer() const
     return "";
 }
 
+// https://html.spec.whatwg.org/multipage/browsers.html#fully-active
+bool Document::is_fully_active() const
+{
+    // A Document d is said to be fully active when d's browsing context is non-null, d's browsing context's active document is d,
+    // and either d's browsing context is a top-level browsing context, or d's browsing context's container document is fully active.
+    return browsing_context() && browsing_context()->active_document() == this && (browsing_context()->is_top_level() || browsing_context()->container_document()->is_fully_active());
+}
+
 }

Userland/Libraries/LibWeb/DOM/Document.h

@@ -274,6 +274,8 @@ public:
 
     bool has_a_style_sheet_that_is_blocking_scripts() const;
 
+    bool is_fully_active() const;
+
 private:
     explicit Document(const URL&);