Make PrincipalHolder threadsafe since it can be destroyed from off-thread (thanks to XPCSafeJSContexts) and no additional work is required to make it threadsafe. bug 337611, r+sr=jst

js/src/xpconnect/src/xpccomponents.cpp

@@ -2961,7 +2961,7 @@ nsXPCComponents_Utils::ReportError()
 #include "nsNetUtil.h"
 const char kScriptSecurityManagerContractID[] = NS_SCRIPTSECURITYMANAGER_CONTRACTID;
 
-NS_IMPL_ISUPPORTS1(PrincipalHolder, nsIScriptObjectPrincipal)
+NS_IMPL_THREADSAFE_ISUPPORTS1(PrincipalHolder, nsIScriptObjectPrincipal)
 
 nsIPrincipal *
 PrincipalHolder::GetPrincipal()