bind: pin Go objects while incrementing their reference count

When passing a refnum across the language barrier there is a small
window where a proxy object itself can be garbage collected, its
reference count go to 0 and the object be gone when the refnum
is dereferenced on the other side.

In Go the proxy object is pinned with runtime.KeepAlive. This CL
implements the same mechanism in Java by passing the proxy object to
native code, ensuring the Java GC can't reclaim it during the call.

Change-Id: I23824439012eb00f90d729f59d4846999f24f01f
Reviewed-on: https://go-review.googlesource.com/107095
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/java/Seq.java

@@ -98,7 +98,10 @@ public class Seq {
 	}
 
 	// Increment the Go reference count before sending over a refnum.
-	public static native void incGoRef(int refnum);
+	// The ref parameter is only used to make sure the referenced
+	// object is not garbage collected before Go increments the
+	// count. It's the equivalent of Go's runtime.KeepAlive.
+	public static native void incGoRef(int refnum, GoObject ref);
 
 	// Informs the Go ref tracker that Java is done with this refnum.
 	static native void destroyRef(int refnum);