Fix bitfield enums to not trigger upcoming Clang warning

src/base/safe_conversions_impl.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Slightly adapted for inclusion in V8.
 // Copyright 2014 the V8 project authors. All rights reserved.
 
 #ifndef V8_BASE_SAFE_CONVERSIONS_IMPL_H_
 #define V8_BASE_SAFE_CONVERSIONS_IMPL_H_
 
@@ skipping 72 matching lines @@
 
 // Signed to unsigned: Dst cannot be statically determined to contain Src.
 template <typename Dst, typename Src>
 struct StaticDstRangeRelationToSrcRange<Dst,
                                         Src,
                                         INTEGER_REPRESENTATION_UNSIGNED,
                                         INTEGER_REPRESENTATION_SIGNED> {
   static const NumericRangeRepresentation value = NUMERIC_RANGE_NOT_CONTAINED;
 };
 
-enum RangeConstraint {
-  RANGE_VALID = 0x0,  // Value can be represented by the destination type.
+enum RangeConstraint : unsigned char {
+  RANGE_VALID = 0x0,      // Value can be represented by the destination type.
   RANGE_UNDERFLOW = 0x1,  // Value would overflow.
-  RANGE_OVERFLOW = 0x2,  // Value would underflow.
+  RANGE_OVERFLOW = 0x2,   // Value would underflow.
   RANGE_INVALID = RANGE_UNDERFLOW | RANGE_OVERFLOW  // Invalid (i.e. NaN).
 };
 
 // Helper function for coercing an int back to a RangeContraint.
 inline RangeConstraint GetRangeConstraint(int integer_range_constraint) {
   DCHECK(integer_range_constraint >= RANGE_VALID &&
          integer_range_constraint <= RANGE_INVALID);
   return static_cast<RangeConstraint>(integer_range_constraint);
 }
 
@@ skipping 104 matching lines @@
   STATIC_ASSERT(std::numeric_limits<Src>::is_specialized);
   STATIC_ASSERT(std::numeric_limits<Dst>::is_specialized);
   return DstRangeRelationToSrcRangeImpl<Dst, Src>::Check(value);
 }
 
 }  // namespace internal
 }  // namespace base
 }  // namespace v8
 
 #endif  // V8_BASE_SAFE_CONVERSIONS_IMPL_H_