When accessing the "prototype" property of a known JSFunction in
TurboFan, we can automatically create the initial map if it hasn't been
instantiated so far. This happens with hand-grown inheritance mechanisms
where the base classes might not be instantiated regularly at all, but
the base class constructors are only called via F.p.call from the
subclass constructors.

R=mstarzinger@chromium.org
BUG=v8:5267

Review-Url: https://codereview.chromium.org/2830633002
Cr-Commit-Position: refs/heads/master@{#44720}