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/HTMLImageElement.cpp

@@ -67,8 +67,7 @@ static bool IsPreviousSibling(nsINode* aSubject, nsINode* aNode) {
 }
 #endif
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 // Calls LoadSelectedImage on host element unless it has been superseded or
 // canceled -- this is the synchronous section of "update the image data".
@@ -1289,5 +1288,4 @@ void HTMLImageElement::StopLazyLoadingAndStartLoadIfNeeded() {
   StartLoadingIfNeeded();
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom