Disable flaky DemoExtensionsExternalLoaderTest.LoadApp on Linux

TBR=wzang@chromium.org

Bug: 904644
Change-Id: I8cd86ab57a5a5fc782447569e933f095d34a8d4d
Reviewed-on: https://chromium-review.googlesource.com/c/1344049Reviewed-by: Maxim Kolosovskiy <kolos@chromium.org>
Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609689}

chrome/browser/chromeos/login/demo_mode/demo_extensions_external_loader_unittest.cc

@@ -22,6 +22,7 @@
 #include "base/run_loop.h"
 #include "base/values.h"
 #include "base/version.h"
+#include "build/build_config.h"
 #include "chrome/browser/chromeos/login/demo_mode/demo_mode_test_helper.h"
 #include "chrome/browser/chromeos/login/demo_mode/demo_session.h"
 #include "chrome/browser/chromeos/login/users/fake_chrome_user_manager.h"
@@ -440,6 +441,11 @@ TEST_F(DemoExtensionsExternalLoaderTest,
   EXPECT_TRUE(external_provider_visitor_.loaded_crx_files().empty());
 }
 
+#if defined(OS_LINUX)
+#define MAYBE_LoadApp DISABLED_LoadApp
+#else
+#define MAYBE_LoadApp LoadApp
+#endif
 TEST_F(DemoExtensionsExternalLoaderTest, LoadApp) {
   demo_mode_test_helper_->InitializeSession();