X87: Fix computed properties on object literals with a double as propertyname.

port r22255.

original commit message:

  Fix computed properties on object literals with a double as propertyname.

BUG=
R=weiliang.lin@intel.com

Review URL: https://codereview.chromium.org/379513002

Patch from Chunyang Dai <chunyang.dai@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22261 ce2b1a6d-e550-0410-aec6-3dcde31c8c00