Bug 1637869 - P4. Access pref via staticpref. r=mattwoodrow,necko-reviewers

It would otherwise triggers browser/base/content/test/performance/browser_preferences_usage.js

That pref would have been checked with every single load, a staticpref is O-1 .

Differential Revision: https://phabricator.services.mozilla.com/D76317

docshell/base/nsDocShell.cpp

@@ -8960,7 +8960,7 @@ bool nsDocShell::CanLoadInParentProcess(nsIURI* aURI) {
   }
   // Final exception for some legacy automated tests:
   if (xpc::IsInAutomation() &&
-      Preferences::GetBool("security.allow_unsafe_parent_loads", false)) {
+      StaticPrefs::security_allow_unsafe_parent_loads()) {
     return true;
   }
   return false;