Backport hash collision workaround to 3.6.

src/heap.h

Index: src/heap.h
===================================================================
--- src/heap.h	(revision 10353)
+++ src/heap.h	(working copy)
@@ -79,6 +79,7 @@
   V(FixedArray, single_character_string_cache, SingleCharacterStringCache)     \
   V(FixedArray, string_split_cache, StringSplitCache)                          \
   V(Object, termination_exception, TerminationException)                       \
+  V(Smi, string_hash_seed, StringHashSeed)                                     \
   V(FixedArray, empty_fixed_array, EmptyFixedArray)                            \
   V(ByteArray, empty_byte_array, EmptyByteArray)                               \
   V(FixedDoubleArray, empty_fixed_double_array, EmptyFixedDoubleArray)         \
@@ -841,8 +842,7 @@
   // Please note this function does not perform a garbage collection.
   MUST_USE_RESULT MaybeObject* LookupSymbol(Vector<const char> str);
   MUST_USE_RESULT MaybeObject* LookupAsciiSymbol(Vector<const char> str);
-  MUST_USE_RESULT MaybeObject* LookupTwoByteSymbol(
-      Vector<const uc16> str);
+  MUST_USE_RESULT MaybeObject* LookupTwoByteSymbol(Vector<const uc16> str);
   MUST_USE_RESULT MaybeObject* LookupAsciiSymbol(const char* str) {
     return LookupSymbol(CStrVector(str));
   }
@@ -1301,6 +1301,12 @@
     if (global_gc_epilogue_callback_ != NULL) global_gc_epilogue_callback_();
   }
 
+  uint32_t StringHashSeed() {
+    uint32_t seed = static_cast<uint32_t>(string_hash_seed()->value());
+    ASSERT(FLAG_randomize_string_hashes || seed == 0);
+    return seed;
+  }
+
  private:
   Heap();