Inhibit client-side health checking for backends from balancer in xds.

src/core/ext/filters/client_channel/lb_policy/xds/xds.cc

@@ -1307,11 +1307,14 @@ grpc_channel_args* XdsLb::CreateChildPolicyArgsLocked() {
       grpc_channel_arg_integer_create(
           const_cast<char*>(GRPC_ARG_ADDRESS_IS_BACKEND_FROM_XDS_LOAD_BALANCER),
           1),
+      // Inhibit client-side health checking, since the balancer does
+      // this for us.
+      grpc_channel_arg_integer_create(
+          const_cast<char*>(GRPC_ARG_INHIBIT_HEALTH_CHECKING), 1),
   };
-  grpc_channel_args* args = grpc_channel_args_copy_and_add_and_remove(
+  return grpc_channel_args_copy_and_add_and_remove(
       args_, keys_to_remove, GPR_ARRAY_SIZE(keys_to_remove), args_to_add,
       GPR_ARRAY_SIZE(args_to_add));
-  return args;
 }
 
 void XdsLb::CreateChildPolicyLocked(const char* name, Args args) {