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.

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

R=rossberg@chromium.org

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

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#24990}
git-svn-id: f98b9d40cb02301bc76fa9e1b46ee668158567fd@24990 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

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