Desktop capture: Fix GN target and exclude differ* sources on iOS

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("../../build/webrtc.gni")
@@ skipping 20 matching lines @@
     "cropped_desktop_frame.cc",
     "cropped_desktop_frame.h",
     "cropping_window_capturer.cc",
     "cropping_window_capturer.h",
     "cropping_window_capturer_win.cc",
     "desktop_and_cursor_composer.cc",
     "desktop_and_cursor_composer.h",
     "desktop_capture_options.cc",
     "desktop_capture_options.h",
     "desktop_capturer.h",
-    "desktop_capturer.h",
     "desktop_frame_win.cc",
     "desktop_frame_win.h",
-    "differ.cc",
-    "differ.h",
-    "differ_block.cc",
-    "differ_block.h",
     "mac/desktop_configuration.h",
     "mac/desktop_configuration.mm",
     "mac/desktop_configuration_monitor.cc",
     "mac/desktop_configuration_monitor.h",
     "mac/full_screen_chrome_window_detector.cc",
     "mac/full_screen_chrome_window_detector.h",
     "mac/scoped_pixel_buffer_object.cc",
     "mac/scoped_pixel_buffer_object.h",
     "mac/window_list_utils.cc",
     "mac/window_list_utils.h",
@@ skipping 64 matching lines @@
   }
 
   if (!is_win && !is_mac && !use_x11) {
     sources += [
       "mouse_cursor_monitor_null.cc",
       "screen_capturer_null.cc",
       "window_capturer_null.cc",
     ]
   }
 
+  if (!is_ios) {
+    sources += [
+      "differ.cc",
+      "differ.h",
+      "differ_block.cc",
+      "differ_block.h",
+    ]
+  }
+
   if (is_mac) {
     libs = [
       "AppKit.framework",
       "IOKit.framework",
       "OpenGL.framework",
     ]
   }
 
   if (is_win) {
     libs = [
@@ skipping 33 matching lines @@
     ]
 
     configs += [ "../..:common_config" ]
     public_configs = [ "../..:common_inherited_config" ]
 
     if (is_posix) {
       cflags = [ "-msse2" ]
     }
   }
 }