Bug 1615141 - Fix panic in picture cache dependency building. r=Bert
Bug 1615141 - Fix panic in picture cache dependency building. r=Bert

Differential Revision: https://phabricator.services.mozilla.com/D62851

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/fc74126fedeb2bb945db4d52e54266c955e9bbe7

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