Only allow one thread to abort at a time.

Found when looking to see if Google Earth had fixed their JNI bug yet.

Change-Id: I8768248625869d344ba04ecf4fec06047f76c3ce
diff --git a/src/runtime.h b/src/runtime.h
index 2b3fd7e..f950add 100644
--- a/src/runtime.h
+++ b/src/runtime.h
@@ -285,6 +285,8 @@
   // A pointer to the active runtime or NULL.
   static Runtime* instance_;
 
+  static Mutex abort_lock_;
+
   DISALLOW_COPY_AND_ASSIGN(Runtime);
 };