LibWeb: Add missing calls to Base::children_changed

Also making sure that it is called at the top of the implementation
for consistency.

Libraries/LibWeb/HTML/HTMLMediaElement.cpp

@@ -768,6 +768,8 @@ GC_DEFINE_ALLOCATOR(SourceElementSelector);
 
 void HTMLMediaElement::children_changed()
 {
+    Base::children_changed();
+
     if (m_source_element_selector)
         m_source_element_selector->process_next_candidate().release_value_but_fixme_should_propagate_errors();
 }

Libraries/LibWeb/HTML/HTMLScriptElement.cpp

@@ -574,6 +574,8 @@ void HTMLScriptElement::prepare_script()
 // https://html.spec.whatwg.org/multipage/scripting.html#script-processing-model:html-element-post-connection-steps-4
 void HTMLScriptElement::children_changed()
 {
+    Base::children_changed();
+
     // 1. Run the script HTML element post-connection steps, given the script element.
     post_connection();
 }

Libraries/LibWeb/HTML/HTMLStyleElement.cpp

@@ -34,8 +34,8 @@ void HTMLStyleElement::visit_edges(Cell::Visitor& visitor)
 
 void HTMLStyleElement::children_changed()
 {
-    m_style_element_utils.update_a_style_block(*this);
     Base::children_changed();
+    m_style_element_utils.update_a_style_block(*this);
 }
 
 void HTMLStyleElement::inserted()

Libraries/LibWeb/HTML/HTMLTextAreaElement.cpp

@@ -431,6 +431,8 @@ void HTMLTextAreaElement::update_placeholder_visibility()
 // https://html.spec.whatwg.org/multipage/form-elements.html#the-textarea-element:children-changed-steps
 void HTMLTextAreaElement::children_changed()
 {
+    Base::children_changed();
+
     // The children changed steps for textarea elements must, if the element's dirty value flag is false,
     // set the element's raw value to its child text content.
     if (!m_dirty_value) {

Libraries/LibWeb/SVG/SVGStyleElement.cpp

@@ -32,8 +32,8 @@ void SVGStyleElement::visit_edges(Cell::Visitor& visitor)
 
 void SVGStyleElement::children_changed()
 {
-    m_style_element_utils.update_a_style_block(*this);
     Base::children_changed();
+    m_style_element_utils.update_a_style_block(*this);
 }
 
 void SVGStyleElement::inserted()