Use a random seed for the string hash algorithm.

src/isolate.h

Index: src/isolate.h
===================================================================
--- src/isolate.h	(revision 10321)
+++ src/isolate.h	(working copy)
@@ -913,6 +913,13 @@
     return &interp_canonicalize_mapping_;
   }
 
+  uint32_t HasherSeed() {
+    if (FLAG_randomize_string_hashes) {
+      ASSERT(hasher_seed_ != 0);
+    }
+    return hasher_seed_;
+  }
+
   void* PreallocatedStorageNew(size_t size);
   void PreallocatedStorageDelete(void* p);
   void PreallocatedStorageInit(size_t size);
@@ -1174,6 +1181,7 @@
   RegExpStack* regexp_stack_;
   unibrow::Mapping<unibrow::Ecma262Canonicalize> interp_canonicalize_mapping_;
   void* embedder_data_;
+  uint32_t hasher_seed_;
 
 #if defined(V8_TARGET_ARCH_ARM) && !defined(__arm__) || \
     defined(V8_TARGET_ARCH_MIPS) && !defined(__mips__)