The %_ClassOf intrinsic roughly corresponds to the deprecated ES5
[[Class]] internal property, and should not be used anymore ideally.
However since we still have quite a couple of uses of this intrinsic
in the self hosted JavaScript builtins, we would tank some builtins
like Map, Set, WeakMap, WeakSet, etc. quite significantly unless we
also support this intrinsic until the builtins are all migrated to
C++/CSA builtins.

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

Review-Url: https://codereview.chromium.org/2647833004
Cr-Commit-Position: refs/heads/master@{#42530}