Bug 1510030 - Make prepare_prim_for_render private. r=gw
Bug 1510030 - Make prepare_prim_for_render private. r=gw

Differential Revision: https://phabricator.services.mozilla.com/D63603

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/cd729f8b02207a2e44396e7886fbeb7d97543c75

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