Bug 1451615: Whitelist harmless rejection. r=me,test-only DONTBUILD

MozReview-Commit-ID: Cq3ybb4rKgF

toolkit/components/extensions/test/xpcshell/test_ext_redirects.js

@@ -5,6 +5,8 @@ ChromeUtils.defineModuleGetter(this, "TestUtils",
                                "resource://testing-common/TestUtils.jsm");
 Cu.importGlobalProperties(["XMLHttpRequest"]);
 
+PromiseTestUtils.whitelistRejectionsGlobally(/Message manager disconnected/);
+
 const server = createHttpServer();
 const gServerUrl = `http://localhost:${server.identity.primaryPort}`;