Use ThreadChecker rather than DCHECK_CURRENTLY_ON.

chrome/browser/content_settings/host_content_settings_map.h

Index: chrome/browser/content_settings/host_content_settings_map.h
diff --git a/chrome/browser/content_settings/host_content_settings_map.h b/chrome/browser/content_settings/host_content_settings_map.h
index 9316b6f07c657aff4dc06f5df8011ee27cf68660..d6aa51834dea8c6a989e12c2728c079fa409dcae 100644
--- a/chrome/browser/content_settings/host_content_settings_map.h
+++ b/chrome/browser/content_settings/host_content_settings_map.h
@@ -17,6 +17,7 @@
 #include "base/observer_list.h"
 #include "base/prefs/pref_change_registrar.h"
 #include "base/threading/platform_thread.h"
+#include "base/threading/thread_checker.h"
 #include "base/tuple.h"
 #include "chrome/browser/content_settings/content_settings_override_provider.h"
 #include "components/content_settings/core/browser/content_settings_observer.h"
@@ -61,6 +62,8 @@ class HostContentSettingsMap
     NUM_PROVIDER_TYPES,
   };
 
+  // This should be called on the UI thread, otherwise |thread_checker_| handles
+  // CalledOnValidThread() wrongly.
   HostContentSettingsMap(PrefService* prefs, bool incognito);
 
   static void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry);
@@ -354,6 +357,8 @@ class HostContentSettingsMap
   // before any other uses of it.
   ProviderMap content_settings_providers_;
 
+  base::ThreadChecker thread_checker_;
+
   ObserverList<content_settings::Observer> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(HostContentSettingsMap);