Improve Math.round(). Fix the bug in r4146. Further improve performance by checking the exponent instead of comparing doubles. Add several tests for numbers near the limits of SMI and several tests from WebKit.

Review URL: http://codereview.chromium.org/1008004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4180 ce2b1a6d-e550-0410-aec6-3dcde31c8c00