Add "webviewupdate" system server service.

Define the service context for "webviewupdate", a new service that will
run in the system server.

Bug: 13005501
Change-Id: I841437c59b362fda88d130be2f2871aef87d9231

service_contexts

@@ -111,6 +111,7 @@ user                                      u:object_r:system_server_service:s0
 vibrator                                  u:object_r:system_server_service:s0
 voiceinteraction                          u:object_r:system_server_service:s0
 wallpaper                                 u:object_r:system_server_service:s0
+webviewupdate                             u:object_r:system_server_service:s0
 wifip2p                                   u:object_r:system_server_service:s0
 wifiscanner                               u:object_r:system_server_service:s0
 wifi                                      u:object_r:system_server_service:s0