Speed up creation of Objects whose prototype has dictionary elements

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