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/TextTrackManager.cpp

@@ -30,8 +30,7 @@ mozilla::LazyLogModule gTextTrackLog("WebVTT");
   MOZ_LOG(gTextTrackLog, LogLevel::Verbose, \
           ("TextTrackManager=%p, " msg, this, ##__VA_ARGS__))
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 NS_IMPL_ISUPPORTS(TextTrackManager::ShutdownObserverProxy, nsIObserver);
 
@@ -875,5 +874,4 @@ void TextTrackManager::MaybeRunTimeMarchesOn() {
   TimeMarchesOn();
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom