Use signaling NaN for holes in fixed double arrays.

test/cctest/test-serialize.cc

Index: test/cctest/test-serialize.cc
diff --git a/test/cctest/test-serialize.cc b/test/cctest/test-serialize.cc
index 1f7eb37fda1775c4d1f97ec81df486cfa5c2d08d..4c6e6231d6cff658d8242b2ef42fcc198ab20046 100644
--- a/test/cctest/test-serialize.cc
+++ b/test/cctest/test-serialize.cc
@@ -88,7 +88,7 @@ TEST(ExternalReferenceEncoder) {
   CHECK_EQ(
       make_code(UNCLASSIFIED, 1),
       encoder.Encode(ExternalReference::roots_array_start(isolate).address()));
-  CHECK_EQ(make_code(UNCLASSIFIED, 34),
+  CHECK_EQ(make_code(UNCLASSIFIED, 33),
            encoder.Encode(ExternalReference::cpu_features().address()));
 }