Return to previous random number generator, but mix more bits into
output to hide hidden state better.  Keep the multithreading fix that
moves the PNG into isolate.

BUG=
TEST=

Review URL: http://codereview.chromium.org/7250005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8524 ce2b1a6d-e550-0410-aec6-3dcde31c8c00