Simplify, speed-up correct-context ObjectObserve calls

The original patch which ensured that Object.observe did allocations in the correct context regressed performance about 12%. This patch gets back most of that (about 11%) by simply returning the correct function which is then directly callable from JS, rather than by making the call from the runtime function. A side-effect is that their implementation is shorter.

LOG=Y
BUG=NONE
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/307543008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21575 ce2b1a6d-e550-0410-aec6-3dcde31c8c00