servo: Merge #17980 - Introduce a new flag and use it to be more permissive about cousin sharing (from bholley:fix_initial_sharing); r=emilio

https://bugzilla.mozilla.org/show_bug.cgi?id=1387116

Source-Repo: https://github.com/servo/servo
Source-Revision: b701d726f2840d61573d213f2e2008335dc9831e

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 4f81898b45e85740b61ae598a7f855708d8c0f2b