Remove DCHECK in DialRegistry dtor. This may trigger on browser shutdown if

there are event listeners still registered with chrome.dial.  This is not an
issue as all owned objects will be cleaned up anyway by following the normal dtor
sequence DialAPI -> DialRegistry -> DialService.

BUG=460197

Review URL: https://codereview.chromium.org/916103007

Cr-Commit-Position: refs/heads/master@{#317194}

chrome/browser/extensions/api/dial/dial_api.h

@@ -21,6 +21,10 @@ class DialRegistry;
 // the DIAL registry. It takes care of creating the registry on the IO thread
 // and is an observer of the registry. It makes sure devices events are sent out
 // to extension listeners on the right thread.
+//
+// TODO(mfoltz): This should probably inherit from BrowserContextKeyedAPI
+// instead; ShutdownOnUIThread below is a no-op, which is the whole point of
+// RefcountedKeyedService.
 class DialAPI : public RefcountedKeyedService,
                 public EventRouter::Observer,
                 public DialRegistry::Observer {

chrome/browser/extensions/api/dial/dial_registry.cc

@@ -42,7 +42,6 @@ DialRegistry::DialRegistry(Observer* dial_api,
 
 DialRegistry::~DialRegistry() {
   DCHECK(thread_checker_.CalledOnValidThread());
-  DCHECK_EQ(0, num_listeners_);
   NetworkChangeNotifier::RemoveNetworkChangeObserver(this);
 }