Bug 1443925 - Part 5: Make it possible to get the system principal from any thread, r=ckerschb

This is required because the script security manager which currently owns the
singleton is main-thread only. This change still ties the lifecycle of the
static to that service, but also makes it generally available from any thread.

Differential Revision: https://phabricator.services.mozilla.com/D163035

caps/nsScriptSecurityManager.cpp

@@ -1554,9 +1554,7 @@ nsresult nsScriptSecurityManager::Init() {
   InitPrefs();
 
   // Create our system principal singleton
-  RefPtr<SystemPrincipal> system = SystemPrincipal::Create();
-
-  mSystemPrincipal = system;
+  mSystemPrincipal = SystemPrincipal::Init();
 
   return NS_OK;
 }
@@ -1600,6 +1598,7 @@ nsScriptSecurityManager::~nsScriptSecurityManager(void) {
 void nsScriptSecurityManager::Shutdown() {
   NS_IF_RELEASE(sIOService);
   BundleHelper::Shutdown();
+  SystemPrincipal::Shutdown();
 }
 
 nsScriptSecurityManager* nsScriptSecurityManager::GetScriptSecurityManager() {