For an object literal, has_seen_proto is needed to create the
BoilerplateDescription. When iterating over the object
properties in the AST, has_seen_proto can easily be computed. The
flag in the ObjectLiteral is unnecessary.

R=verwaest@chromium.org

BUG=v8:5625

Review-Url: https://codereview.chromium.org/2646333002
Cr-Commit-Position: refs/heads/master@{#42601}