Implement generator mirror

R=yangguo@chromium.org, aandrey@chromium.org

BUG=v8:3292
LOG=N

Review URL: https://codereview.chromium.org/580823002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24043 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Avoid idle times smaller than 1ms.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/580083002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24044 ce2b1a6d-e550-0410-aec6-3dcde31c8c00