Use a random seed for the string hash algorithm.

src/x64/code-stubs-x64.cc

Index: src/x64/code-stubs-x64.cc
===================================================================
--- src/x64/code-stubs-x64.cc	(revision 10321)
+++ src/x64/code-stubs-x64.cc	(working copy)
@@ -4958,10 +4958,20 @@
                                     Register hash,
                                     Register character,
                                     Register scratch) {
-  // hash = character + (character << 10);
-  __ movl(hash, character);
-  __ shll(hash, Immediate(10));
-  __ addl(hash, character);
+  int32_t seed = masm->isolate()->HasherSeed();
+  if (seed != 0) {
+    ASSERT(FLAG_randomize_string_hashes);
+    // hash = (seed + character) + ((seed + character) << 10);
+    __ leal(scratch, Operand(character, seed));
+    __ shll(scratch, Immediate(10));
+    __ leal(hash, Operand(scratch, character, times_1, seed));
+  } else {
+    ASSERT(!FLAG_randomize_string_hashes);
+    // hash = character + (character << 10);
+    __ movl(hash, character);
+    __ shll(hash, Immediate(10));
+    __ addl(hash, character);
+  }
   // hash ^= hash >> 6;
   __ movl(scratch, hash);
   __ shrl(scratch, Immediate(6));