Content Settings: Add RevocationObserver to measure when permissions are revoked

components/content_settings/core/browser/content_settings_observer.h

Index: components/content_settings/core/browser/content_settings_observer.h
diff --git a/components/content_settings/core/browser/content_settings_observer.h b/components/content_settings/core/browser/content_settings_observer.h
index 41868578942081c5020c86846319318f367aa47d..2c2cc900ca8bf57b194fc26bfedde4d7343566eb 100644
--- a/components/content_settings/core/browser/content_settings_observer.h
+++ b/components/content_settings/core/browser/content_settings_observer.h
@@ -20,6 +20,15 @@ class Observer {
       ContentSettingsType content_type,
       std::string resource_identifier) = 0;
 
+  // Fired when the content setting for an origin pair is changed from ALLOW to
+  // BLOCK or ASK. Only called when the user's preference is explicitly changed
+  // via HostContentSettingsMap::SetContentSettingDefaultScope().
+  virtual void OnContentSettingRevoked(
+      const GURL& primary_url,
+      const GURL& secondary_url,
+      ContentSettingsType content_type,
+      std::string resource_identifier) {};
+
  protected:
   virtual ~Observer() {}
 };