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/HTMLIFrameElement.cpp

@@ -20,8 +20,7 @@
 
 NS_IMPL_NS_NEW_HTML_ELEMENT_CHECK_PARSER(IFrame)
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 NS_IMPL_CYCLE_COLLECTION_CLASS(HTMLIFrameElement)
 
@@ -303,5 +302,4 @@ void HTMLIFrameElement::RefreshFeaturePolicy(bool aParseAllowAttribute) {
   MaybeStoreCrossOriginFeaturePolicy();
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom