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.

This is a reland of r24972, retaining GetObservationStateJS() to
keep snapshotting working properly.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/663253006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24990 ce2b1a6d-e550-0410-aec6-3dcde31c8c00