Remove extra sizeof().

Improves code readability.

Test: device boots
Change-Id: Ia1e834b57e42343dcc5e8005a1c5b03196db50c0
diff --git a/runtime/obj_ptr.h b/runtime/obj_ptr.h
index 70e767a..14fdba3 100644
--- a/runtime/obj_ptr.h
+++ b/runtime/obj_ptr.h
@@ -37,7 +37,7 @@
 template<class MirrorType>
 class ObjPtr {
   static constexpr size_t kCookieShift =
-      sizeof(kHeapReferenceSize) * kBitsPerByte - kObjectAlignmentShift;
+      kHeapReferenceSize * kBitsPerByte - kObjectAlignmentShift;
   static constexpr size_t kCookieBits = sizeof(uintptr_t) * kBitsPerByte - kCookieShift;
   static constexpr uintptr_t kCookieMask = (static_cast<uintptr_t>(1u) << kCookieBits) - 1;