Reapply the non-MathExtras parts of

Source/wtf/CheckedArithmetic.h

Index: Source/wtf/CheckedArithmetic.h
diff --git a/Source/wtf/CheckedArithmetic.h b/Source/wtf/CheckedArithmetic.h
index 75d4a41b418f508166fca6e4a3fbfc5f0ffc4329..dcbc896b8ad81217d88ff96e8038ea44efdd3417 100644
--- a/Source/wtf/CheckedArithmetic.h
+++ b/Source/wtf/CheckedArithmetic.h
@@ -251,7 +251,7 @@ template <typename LHS, typename RHS, typename ResultType> struct ArithmeticOper
                     return false;
             }
         } // if the signs do not match this operation can't overflow
-        result = lhs + rhs;
+        result = static_cast<ResultType>(lhs + rhs);
         return true;
     }
 
@@ -266,7 +266,7 @@ template <typename LHS, typename RHS, typename ResultType> struct ArithmeticOper
                     return false;
             }
         } // if the signs match this operation can't overflow
-        result = lhs - rhs;
+        result = static_cast<ResultType>(lhs - rhs);
         return true;
     }
 
@@ -291,7 +291,7 @@ template <typename LHS, typename RHS, typename ResultType> struct ArithmeticOper
                     return false;
             }
         }
-        result = lhs * rhs;
+        result = static_cast<ResultType>(lhs * rhs);
         return true;
     }