The bug in the existing code was that it modified the |attributes|
local variable on its way through the loop in CopyUpToAddAttributes.
But that affected any properties updated after an accessor property.
The code now sets up a mask each time and applies that instead of
mutating |attributes|.

R=verwaest@chromium.org

Review URL: https://chromiumcodereview.appspot.com/16051002

Patch from Adam Klein <adamk@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14818 ce2b1a6d-e550-0410-aec6-3dcde31c8c00