Reland "Enable Object.observe by default" again

This re-re-re-lands enabling Object.observe. The Chromium tests that
failed last time this was rolled into Chromium have been disabled in
https://src.chromium.org/viewvc/chrome?view=revision&revision=256706

This patch should be safe to merge once that lands.

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

Review URL: https://codereview.chromium.org/198383002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19868 ce2b1a6d-e550-0410-aec6-3dcde31c8c00