Fix desktop-id notification edge case (#12216)

* Fix desktop-id notification edge case

* Extract-method platform_util::GetDesktopName()

This removes duplicated code from libnotify_notifications.cc
and atom/common/linux/application_info.cc.

* Check for empty case in GetDesktopName().

* Move GetDesktopName() to brightray::util

* Remove unnecessary changes in platform_util

* Add a brightray::platform_util namespace

brightray/common/platform_util.h

@@ -0,0 +1,24 @@
+// Copyright (c) 2018 GitHub, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#ifndef BRIGHTRAY_COMMON_PLATFORM_UTIL_H_
+#define BRIGHTRAY_COMMON_PLATFORM_UTIL_H_
+
+#include <string>
+
+namespace brightray {
+
+namespace platform_util {
+
+#if defined(OS_LINUX)
+// Returns a success flag.
+// Unlike libgtkui, does *not* use "chromium-browser.desktop" as a fallback.
+bool GetDesktopName(std::string* setme);
+#endif
+
+}  // namespace platform_util
+
+}  // namespace brightray
+
+#endif  // BRIGHTRAY_COMMON_PLATFORM_UTIL_H_