Bug 1674637 - Use nested namespaces in dom/ r=sg,andi

Also add some missing namespace qualifications

Done with:
./mach static-analysis check --checks="-*,modernize-concat-nested-namespaces" --fix dom/
./mach clang-format -p $(hg status dom/|grep ^M|awk '{print $2}')

Differential Revision: https://phabricator.services.mozilla.com/D95456

dom/html/HTMLTextAreaElement.cpp

@@ -45,8 +45,7 @@
 
 NS_IMPL_NS_NEW_HTML_ELEMENT_CHECK_PARSER(TextArea)
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 HTMLTextAreaElement::HTMLTextAreaElement(
     already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
@@ -1178,5 +1177,4 @@ void HTMLTextAreaElement::GetAutocomplete(DOMString& aValue) {
       attributeVal, aValue, mAutocompleteAttrState);
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom