Add default allowed value of Ask for Push Notifications.

Push Notifications not only support Ask already, it is the default value, so this should match that reality.

BUG=425698

Review URL: https://codereview.chromium.org/689993002

Cr-Commit-Position: refs/heads/master@{#302136}

components/content_settings/core/browser/host_content_settings_map.cc

@@ -525,6 +525,7 @@ bool HostContentSettingsMap::IsSettingAllowedForType(
     case CONTENT_SETTINGS_TYPE_PPAPI_BROKER:
     case CONTENT_SETTINGS_TYPE_AUTOMATIC_DOWNLOADS:
     case CONTENT_SETTINGS_TYPE_MIDI_SYSEX:
+    case CONTENT_SETTINGS_TYPE_PUSH_MESSAGING:
       return setting == CONTENT_SETTING_ASK;
     default:
       return false;