Remove Chrome OS-only sources only if the OS is Chrome OS.

chrome/chrome_tests_unit.gypi

 # Copyright 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 {
   'variables': {
     'chrome_unit_tests_sources': [
       '../components/autofill/content/renderer/test_password_autofill_agent.cc',
       '../components/autofill/content/renderer/test_password_autofill_agent.h',
       '../components/autofill/content/renderer/test_password_generation_agent.cc',
       '../components/autofill/content/renderer/test_password_generation_agent.h',
@@ skipping 2328 matching lines @@
         ['enable_captive_portal_detection==1', {
           'sources': [ '<@(chrome_unit_tests_captive_portal_sources)' ],
         }],
         ['enable_session_service==1', {
           'sources': [ '<@(chrome_unit_tests_session_service_sources)' ],
         }],
         ['enable_webrtc==1', {
           'sources': [ '<@(chrome_unit_tests_webrtc_sources)' ],
         }],
         ['chromeos==1', {
           'sources': [ '<@(chrome_unit_tests_chromeos_sources)' ],

[wtc, 2015/03/13 00:42:54]

This 'chromeos==1' section is the right place to remove
event_rewriter_unittest.cc because it is part of
chrome_unit_tests_chromeos_sources.

           'dependencies': [
             '../ash/ash_resources.gyp:ash_resources',
             '../ui/chromeos/ui_chromeos.gyp:ui_chromeos_resources',
           ],
           'sources!': [
             'browser/signin/signin_global_error_unittest.cc',
             'browser/signin/signin_manager_unittest.cc',
             'browser/signin/signin_names_io_thread_unittest.cc',
             'browser/ui/views/app_list/linux/app_list_linux_unittest.cc',
             'browser/ui/views/frame/opaque_browser_frame_view_layout_unittest.cc',
           ],
+          'conditions': [
+            ['use_ozone==1', {
+              'sources!': [
+                # crbug.com/354036
+                'browser/chromeos/events/event_rewriter_unittest.cc',
+              ],
+            }],
+          ],
         }],
         ['use_x11==1', {
           'dependencies': [
             '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
             '../ui/events/devices/events_devices.gyp:events_devices',
           ],
         }],
         [ 'cld_version==0 or cld_version==2', {
           'dependencies': [
             # Unit tests should be independent of the CLD2 access mechanism,
@@ skipping 221 matching lines @@
           'dependencies': [
             '../third_party/cld/cld.gyp:cld',
           ],
         }],
         ['enable_app_list==1', {
           'sources': [ '<@(chrome_unit_tests_app_list_sources)' ],
           'dependencies': [
             '../ui/app_list/app_list.gyp:app_list_test_support',
           ],
         }],
-        ['use_ozone==1', {
-          'sources!': [
-            # crbug.com/354036
-            'browser/chromeos/events/event_rewriter_unittest.cc',
-          ],
-        }],
         ['enable_plugin_installation==0', {
           'sources!': [
             'browser/plugins/plugin_installer_unittest.cc',
           ],
         }],
         # Adding more conditions? Don't forget to update the GN build.
       ],
     },
     {
       # GN: //chrome/test:chrome_app_unittests
@@ skipping 141 matching lines @@
               'dependencies': [
                 '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
               ],
             }],
           ],
         },
       ],
     }],
   ],  # 'conditions'
 }