Use native fields for process pid and fix bug that allowed killing

runtime/bin/process.h

Index: runtime/bin/process.h
diff --git a/runtime/bin/process.h b/runtime/bin/process.h
index e1157b3a283c8cba2729373e0dfcf9436ca52cad..f3ef5d87cb1281a9835d402c1b98089c25f32fbd 100644
--- a/runtime/bin/process.h
+++ b/runtime/bin/process.h
@@ -36,6 +36,11 @@ class Process {
 
   static intptr_t CurrentProcessId();
 
+  static Dart_Handle GetProcessIdNativeField(Dart_Handle process,
+                                             intptr_t* pid);
+  static Dart_Handle SetProcessIdNativeField(Dart_Handle process,
+                                             intptr_t pid);
+
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Process);
 };