Bug 1549776 - Ensure debug wrench aborts on panic. r=gw
Bug 1549776 - Ensure debug wrench aborts on panic. r=gw

This makes it so that when running reftests, wrench actually terminates
after a panic rather than just hanging. Termination is detectable and so
we can clean up properly instead of waiting until some other layer hits
a timeout.

Differential Revision: https://phabricator.services.mozilla.com/D32010

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/8f4f4cc896abda8635f1f8259c35c408c1c6b8ab

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender