Actually fix build.

src/conversions-inl.h

Index: src/conversions-inl.h
diff --git a/src/conversions-inl.h b/src/conversions-inl.h
index a98680fd80e85ea1ef3a1331974500815c502312..bf2232562e7946f2636d4ae8fcbcdda149a9551c 100644
--- a/src/conversions-inl.h
+++ b/src/conversions-inl.h
@@ -51,6 +51,11 @@ inline double JunkStringValue() {
 }
 
 
+inline double SignedZero(bool negative) {
+  return negative ? uint64_to_double(Double::kSignMask) : 0.0;
+}
+
+
 // The fast double-to-unsigned-int conversion routine does not guarantee
 // rounding towards zero, or any reasonable value if the argument is larger
 // than what fits in an unsigned 32-bit integer.