Revert "Enable Object.observe by default"

This reverts commit r19734 for breeaking ChromiumOS browser tests.
'OpenSpecialTypes/FileManagerBrowserTest.Test/3' started to time out,
bisecting the roll led to this change.
http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%282%29/builds/22224

TBR=rafaelw@chromium.org,rossberg@chromium.org
BUG=v8:2409
LOG=Y

Review URL: https://codereview.chromium.org/195123005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19816 ce2b1a6d-e550-0410-aec6-3dcde31c8c00