Bug 1587448 enable XTCO-nosniff by default r=ckerschb

Differential Revision: https://phabricator.services.mozilla.com/D48709

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

modules/libpref/init/StaticPrefList.yaml

@@ -2091,7 +2091,7 @@
 
 - name: dom.security.respect_document_nosniff
   type: RelaxedAtomicBool
-  value: false
+  value: true
   mirror: always
 
 # Expose the 'policy' attribute in document and HTMLIFrameElement