This fact is depended upon by, at least, Parser::ParseLazy, and quite
likely by other code. There was already code in %FunctionSetName
enforcing this invariant. This patch adds similar code to
Factory::NewSharedFunctionInfo().

BUG=v8:4659
LOG=n

Review URL: https://codereview.chromium.org/1686193003

Cr-Commit-Position: refs/heads/master@{#33914}