Introduce an api to query the microtask autorun state of an isolate

BUG=none
R=adamk@chromium.org, svenpanne@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/264333004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21250 ce2b1a6d-e550-0410-aec6-3dcde31c8c00