Various cleanup/simplification in object-observe.js

Various cleanup/simplification in object-observe.js

The biggest change is the removal of the map wrapper objects: we now operate
directly on the observation weak map, since there are already Get/GetOrCreate/Set
functions for each info map. Various other small cleanups as well, including the
deletion of unnecessary forwarding functions and making use of standard macros.

R=arv@chromium.org, rossberg@chromium.org

Review URL: https://codereview.chromium.org/686773002

Cr-Commit-Position: refs/heads/master@{#24972}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24972 ce2b1a6d-e550-0410-aec6-3dcde31c8c00