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/performance/PerformanceMainThread.cpp

@@ -14,8 +14,7 @@
 #include "mozilla/StaticPrefs_dom.h"
 #include "mozilla/StaticPrefs_privacy.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 namespace {
 
@@ -448,5 +447,4 @@ void PerformanceMainThread::GetEntriesByName(
   }
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom