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/HTMLSelectElement.cpp

@@ -38,8 +38,7 @@
 
 NS_IMPL_NS_NEW_HTML_ELEMENT_CHECK_PARSER(Select)
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 //----------------------------------------------------------------------
 //
@@ -1627,5 +1626,4 @@ JSObject* HTMLSelectElement::WrapNode(JSContext* aCx,
   return HTMLSelectElement_Binding::Wrap(aCx, this, aGivenProto);
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom