Bug 1525036 - Add pref for Cross-Origin policy r=nika

Differential Revision: https://phabricator.services.mozilla.com/D21417

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

modules/libpref/init/StaticPrefList.h

@@ -2096,6 +2096,13 @@ VARCACHE_PREF(
   bool, true
 )
 
+// When this pref is enabled document loads with a mismatched
+// Cross-Origin header will fail to load
+VARCACHE_PREF("browser.tabs.remote.useCrossOriginPolicy",
+              browser_tabs_remote_useCrossOriginPolicy,
+              bool, false
+)
+
 // Prevent system colors from being exposed to CSS or canvas.
 VARCACHE_PREF(
   "ui.use_standins_for_native_colors",