Add service property "LinkMonitorDisable"

This property is use to disable/enable link monitor on the given
service.

BUG=chromium:415367
TEST=emerge-peppy system_api platform2

Change-Id: Iff9a868faa3bd68538021e16adfc337f215fac87
Reviewed-on: https://chromium-review.googlesource.com/219029
Reviewed-by: Paul Stewart <pstew@chromium.org>
Tested-by: Peter Qiu <zqiu@chromium.org>
Commit-Queue: Peter Qiu <zqiu@chromium.org>
diff --git a/dbus/service_constants.h b/dbus/service_constants.h
index 90e571b..27eb7f0 100644
--- a/dbus/service_constants.h
+++ b/dbus/service_constants.h
@@ -391,6 +391,7 @@
     "PortalDetectionFailedStatus";
 const char kSavedIPConfigProperty[] = "SavedIPConfig";
 const char kStaticIPConfigProperty[] = "StaticIPConfig";
+const char kLinkMonitorDisableProperty[] = "LinkMonitorDisable";
 
 // Flimflam provider property names.
 const char kProviderHostProperty[] = "Provider.Host";