Serializer: Cache FlagList::Hash result.

src/flags.h

Index: src/flags.h
diff --git a/src/flags.h b/src/flags.h
index 9ec5d30119fd3547283ca94683bae30d80ba08ef..545c172d36794bc58503e8bfeef39ee1261bf34d 100644
--- a/src/flags.h
+++ b/src/flags.h
@@ -58,7 +58,8 @@ class FlagList {
   // Set flags as consequence of being implied by another flag.
   static void EnforceFlagImplications();
 
-  // Hash of current flags (to quickly determine flag changes).
+  // Hash of flags (to quickly determine mismatching flag expectations).
+  // This hash is calculated during V8::Initialize and cached.
   static uint32_t Hash();
 };