Check the size of pid_t in art::mirror::Class.

Ensure that `art::mirror::Class::clinit_thread_id_` (which is a C
`pid_t` value) and `java.lang.Class.clinitThreadId` (which is a Java
32-bit `int` value) have the same size.

Test: mmma art
Change-Id: Ief8e962a3d012be8f0aa966849496fba6a9fb3b8
diff --git a/runtime/mirror/class.h b/runtime/mirror/class.h
index f640d3b..861b3ef 100644
--- a/runtime/mirror/class.h
+++ b/runtime/mirror/class.h
@@ -1427,6 +1427,7 @@
 
   // Tid used to check for recursive <clinit> invocation.
   pid_t clinit_thread_id_;
+  static_assert(sizeof(pid_t) == sizeof(int32_t), "java.lang.Class.clinitThreadId size check");
 
   // ClassDef index in dex file, -1 if no class definition such as an array.
   // TODO: really 16bits