This moves the initialization of [[HomeObject]] for constructors from
the %DefineClass runtime function into the bytecode generator, and
makes it conditional (resolving an old TODO). As part of this refactor,
avoid a load of "prototype" by returning the class prototype from
%DefineClass.

This is one of many steps in moving more of class definition into
bytecode.

R=rmcilroy@chromium.org

Review-Url: https://codereview.chromium.org/2610683003
Cr-Commit-Position: refs/heads/master@{#42072}