When FieldType::None() returns a cast Smi::FromInt(0), which translates
as nullptr, the FieldType::IsNone() check becomes equivalent to
`this == nullptr` which is not allowed by the standard and
therefore optimized away as a false constant by GCC 6.

This has lead to crashes when invoking methods on FieldType::None().

Using a different Smi constant for FieldType::None() makes the compiler
always include a comparison against that value. The choice of these
constants has no effect as they are effectively arbitrary.

BUG=https://github.com/nodejs/node/issues/8310

Review-Url: https://codereview.chromium.org/2292953002
Cr-Commit-Position: refs/heads/master@{#39023}