This makes sure all use-site of {ObjectLiteral::constant_properties} are
adapted to use the correct {BoilerplateDescription} type instead of just
the {FixedArray} base type.

R=bmeurer@chromium.org

Review-Url: https://codereview.chromium.org/2659603003
Cr-Commit-Position: refs/heads/master@{#42738}