Rounding happens when the number exceeds 53 bits of floating point mantissa. Current implemetation ignores digits after some limits. 0x1000000000000081 was rounded to 0x1000000000000100 while 0x100000000000008000001 was rounded to 0x100000000000000000000.

Review URL: http://codereview.chromium.org/1374005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4309 ce2b1a6d-e550-0410-aec6-3dcde31c8c00