LibWeb: Relayout document on CharacterData data change

This can definitely be optimized to avoid full relayouts in many
situations, but for now let's just go for correctness.

Userland/Libraries/LibWeb/DOM/CharacterData.cpp

@@ -25,6 +25,7 @@
  */
 
 #include <LibWeb/DOM/CharacterData.h>
+#include <LibWeb/DOM/Document.h>
 
 namespace Web::DOM {
 
@@ -38,4 +39,13 @@ CharacterData::~CharacterData()
 {
 }
 
+void CharacterData::set_data(String data)
+{
+    if (m_data == data)
+        return;
+    m_data = move(data);
+    // FIXME: This is definitely too aggressive.
+    document().schedule_forced_layout();
+}
+
 }