Update V8 to r4924 as required by WebKit r61871

Change-Id: Ic819dad0c1c9e035b8ffd306c96656ba87c5e85a
diff --git a/src/heap.h b/src/heap.h
index 0db4008..a8f8c34 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -69,10 +69,12 @@
   V(Map, cons_symbol_map, ConsSymbolMap)                                       \
   V(Map, cons_ascii_symbol_map, ConsAsciiSymbolMap)                            \
   V(Map, external_symbol_map, ExternalSymbolMap)                               \
+  V(Map, external_symbol_with_ascii_data_map, ExternalSymbolWithAsciiDataMap)  \
   V(Map, external_ascii_symbol_map, ExternalAsciiSymbolMap)                    \
   V(Map, cons_string_map, ConsStringMap)                                       \
   V(Map, cons_ascii_string_map, ConsAsciiStringMap)                            \
   V(Map, external_string_map, ExternalStringMap)                               \
+  V(Map, external_string_with_ascii_data_map, ExternalStringWithAsciiDataMap)  \
   V(Map, external_ascii_string_map, ExternalAsciiStringMap)                    \
   V(Map, undetectable_string_map, UndetectableStringMap)                       \
   V(Map, undetectable_ascii_string_map, UndetectableAsciiStringMap)            \
@@ -1882,8 +1884,8 @@
   };
   inline static int Hash(const Converter& c) {
     uint32_t hash = (c.integers[0] ^ c.integers[1]);
-    hash ^= hash >> 16;
-    hash ^= hash >> 8;
+    hash ^= static_cast<int32_t>(hash) >> 16;
+    hash ^= static_cast<int32_t>(hash) >> 8;
     return (hash & (kCacheSize - 1));
   }