Fix issue with storing 31-bit bitfield as Smi.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 1c0fa6462e0802414c5b8eff2489d5628c1efe15..1cdfeb33a2b9d095d3ca6b8a208e4e547bcfa256 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -304,8 +304,10 @@ static const ExtraICState kNoExtraICState = 0;
 // Instance size sentinel for objects of variable size.
 const int kVariableSizeSentinel = 0;
 
+// We may store the unsigned bit field as signed Smi value and do not
+// use the sign bit.
 const int kStubMajorKeyBits = 7;
-const int kStubMinorKeyBits = kBitsPerInt - kSmiTagSize - kStubMajorKeyBits;
+const int kStubMinorKeyBits = kSmiValueSize - kStubMajorKeyBits - 1;
 
 // All Maps have a field instance_type containing a InstanceType.
 // It describes the type of the instances.