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/HTMLInputElement.cpp

@@ -121,8 +121,7 @@ NS_IMPL_NS_NEW_HTML_ELEMENT_CHECK_PARSER(Input)
 
 // XXX align=left, hspace, vspace, border? other nav4 attrs
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 // First bits are needed for the control type.
 #define NS_OUTER_ACTIVATE_EVENT (1 << 9)
@@ -7034,7 +7033,6 @@ already_AddRefed<nsINodeList> HTMLInputElement::GetLabels() {
   return nsGenericHTMLElement::Labels();
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #undef NS_ORIGINAL_CHECKED_VALUE