Bug 1163021 - add debug assert that directory service usage happens only on the main thread, r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D45832

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

xpcom/io/nsDirectoryService.cpp

@@ -183,6 +183,8 @@ nsDirectoryService::Get(const char* aProp, const nsIID& aUuid, void** aResult) {
     return NS_ERROR_INVALID_ARG;
   }
 
+  MOZ_ASSERT(NS_IsMainThread(), "Do not call dirsvc::get on non-main threads!");
+
   nsDependentCString key(aProp);
 
   nsCOMPtr<nsIFile> cachedFile = mHashtable.Get(key);