So that we can query the test value in the child process properly.
Note that the APIs used for setting the prefers-reduced-motion value for testing
are only used on Android and MacOSX. As for MacOSX we have a different
machinery (see bug 1486971) to deliver the test value without spinning native
event loop in the child process so the change here is valid only for Android.
Differential Revision: https://phabricator.services.mozilla.com/D18311
--HG--
extra : moz-landing-system : lando