Add the condition variable to System.loadLibrary and implement UnregisterNatives.

Also change PrettyDescriptor now descriptors are String*s rather than StringPiece&s.

Change-Id: Id07affb26038f5f4a3bee4396c65f71d7bc38be3
diff --git a/src/object.h b/src/object.h
index 7004e59..3c7ec49 100644
--- a/src/object.h
+++ b/src/object.h
@@ -635,9 +635,14 @@
   }
 
   void RegisterNative(const void* native_method) {
+    CHECK(native_method != NULL);
     native_method_ = native_method;
   }
 
+  void UnregisterNative() {
+    native_method_ = NULL;
+  }
+
   static MemberOffset NativeMethodOffset() {
     return MemberOffset(OFFSETOF_MEMBER(Method, native_method_));
   }