Add DesktopRectTest for UnionWith() function

webrtc/modules/desktop_capture/BUILD.gn

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("//build/config/ui.gni")
 import("../../webrtc.gni")
@@ skipping 58 matching lines @@
     # gets additional generated targets which would require many lines here to
     # cover (which would be confusing to read and hard to maintain).
     if (!is_android && !is_ios) {
       visibility = [ "//webrtc/modules:modules_unittests" ]
     }
     sources = [
       "blank_detector_desktop_capturer_wrapper_unittest.cc",
       "desktop_and_cursor_composer_unittest.cc",
       "desktop_capturer_differ_wrapper_unittest.cc",
       "desktop_frame_rotation_unittest.cc",
+      "desktop_geometry_unittest.cc",
       "desktop_region_unittest.cc",
       "differ_block_unittest.cc",
       "fallback_desktop_capturer_wrapper_unittest.cc",
       "mouse_cursor_monitor_unittest.cc",
       "rgba_color_unittest.cc",
       "test_utils.cc",
       "test_utils.h",
       "test_utils_unittest.cc",
       "win/cursor_unittest.cc",
       "win/cursor_unittest_resources.h",
@@ skipping 218 matching lines @@
     sources = [
       "differ_vector_sse2.cc",
       "differ_vector_sse2.h",
     ]
 
     if (is_posix) {
       cflags = [ "-msse2" ]
     }
   }
 }