discovery: add close() method to plugin interface

MultiPart: 3/3
Change-Id: I475d980d1fbafa9600d1990a4c484f58457c5dda
diff --git a/impl/google/discovery/plugins/plugin.go b/impl/google/discovery/plugins/plugin.go
index 59ec56d..a4b9609 100644
--- a/impl/google/discovery/plugins/plugin.go
+++ b/impl/google/discovery/plugins/plugin.go
@@ -84,6 +84,13 @@
 	return nil
 }
 
+func (p *plugin) Close() {
+	env, freeFunc := jutil.GetEnv()
+	defer freeFunc()
+
+	jutil.CallVoidMethod(env, p.jPlugin, "close", nil)
+}
+
 func newPluginFactory(env jutil.Env, jPluginClass jutil.Class) func(*context.T, string) (idiscovery.Plugin, error) {
 	return func(ctx *context.T, host string) (idiscovery.Plugin, error) {
 		env, freeFunc := jutil.GetEnv()