Fix bitfield enums to not trigger upcoming Clang warning

src/base/safe_conversions_impl.h

Index: src/base/safe_conversions_impl.h
diff --git a/src/base/safe_conversions_impl.h b/src/base/safe_conversions_impl.h
index 90c8e19353d71150b29da44cd90b717289f75032..aa799082c810522ddc1d31ecf55a28845cb6bc1b 100644
--- a/src/base/safe_conversions_impl.h
+++ b/src/base/safe_conversions_impl.h
@@ -90,10 +90,10 @@ struct StaticDstRangeRelationToSrcRange<Dst,
   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).
 };