Catch unhandled promise rejections in workers.

BUG=393913
R=pfeldman@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/669723002

git-svn-id: svn://svn.chromium.org/blink/trunk@184086 bbb929c8-8fbe-4397-9dbb-9b2b20218538