Run JS micro tasks in the appropriate context.

R=jochen@chromium.org
BUG=385349
LOG=Y

Review URL: https://codereview.chromium.org/332923003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21933 ce2b1a6d-e550-0410-aec6-3dcde31c8c00