Expose the GC under a name that is less collision prone than window.gc.

This also keeps backwards compatibility (window.gc() still works).

BUG=2641

Review URL: https://codereview.chromium.org/14075012

Patch from Marja Hölttä <marja@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14397 ce2b1a6d-e550-0410-aec6-3dcde31c8c00