Bug 1531078: Disable Date jitter in webrtc wpt. r=jib

Depends on D28544

Differential Revision: https://phabricator.services.mozilla.com/D28545

--HG--
extra : moz-landing-system : lando

testing/web-platform/meta/webrtc/__dir__.ini

@@ -1 +1 @@
-prefs: [media.navigator.permission.disabled:true, media.navigator.streams.fake:true]
+prefs: [media.navigator.permission.disabled:true, media.navigator.streams.fake:true, privacy.resistFingerprinting.reduceTimerPrecision.jitter:false, privacy.reduceTimerPrecision:false]