Migrate might_have_observer to empty in net and blink

Bug: 1155308
Change-Id: Ida2d8011c7a7d1ac14aff806d084e524e46d2bd8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628930Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843974}

net/dns/mdns_client_impl.cc

@@ -377,7 +377,7 @@ void MDnsClientImpl::Core::RemoveListener(MDnsListenerImpl* listener) {
   observer_list_iterator->second->RemoveObserver(listener);
 
   // Remove the observer list from the map if it is empty
-  if (!observer_list_iterator->second->might_have_observers()) {
+  if (observer_list_iterator->second->empty()) {
     // Schedule the actual removal for later in case the listener removal
     // happens while iterating over the observer list.
     base::ThreadTaskRunnerHandle::Get()->PostTask(
@@ -388,7 +388,7 @@ void MDnsClientImpl::Core::RemoveListener(MDnsListenerImpl* listener) {
 
 void MDnsClientImpl::Core::CleanupObserverList(const ListenerKey& key) {
   auto found = listeners_.find(key);
-  if (found != listeners_.end() && !found->second->might_have_observers()) {
+  if (found != listeners_.end() && found->second->empty()) {
     listeners_.erase(found);
   }
 }

third_party/blink/renderer/controller/memory_usage_monitor.cc

@@ -79,7 +79,7 @@ void MemoryUsageMonitor::TimerFired() {
   MemoryUsage usage = GetCurrentMemoryUsage();
   for (auto& observer : observers_)
     observer.OnMemoryPing(usage);
-  if (!observers_.might_have_observers())
+  if (observers_.empty())
     StopMonitoring();
 }