Runtime::RunMicrotask should silent return if no pending microtask work (rather than asserting)

R=rossberg@chromium.org, rossberg
BUG=347532

Review URL: https://codereview.chromium.org/181013008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19588 ce2b1a6d-e550-0410-aec6-3dcde31c8c00