This matches Simulator::registers_ and is in line with the style guide.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/12096036
Patch from Hans Wennborg <hans@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13547 ce2b1a6d-e550-0410-aec6-3dcde31c8c00