Speed up creation of Objects whose prototype has dictionary elements

This speeds up both the case from the bug (using Object.create) but also
takes care ofthe "{ __proto__: obj  }" syntax, which was previously (and
erroneously) being treated the same as setting the prototype dynamically
from script using the __proto__ setter or Object.setPrototypeOf.

BUG=chromium:422754
LOG=y
R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/667253002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24814 ce2b1a6d-e550-0410-aec6-3dcde31c8c00