Remove base/network_change_notifier_linux_unittest.cc exclusion.

It seems this file doesn't exist anymore, so the exclusion
rules aren't also necessary. And GN reports error for 
excluding non-existing files.

BUG=None
R=brettw@chromium.org, szym@chromium.org
TEST=None

Review URL: https://codereview.chromium.org/399723002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283655 0039d316-1c4b-4281-b951-d872f2087c98

net/BUILD.gn

@@ -1096,7 +1096,6 @@ test("net_unittests") {
 
   if (is_chromeos) {
     sources -= [
-      "base/network_change_notifier_linux_unittest.cc",
       "proxy/proxy_config_service_linux_unittest.cc",
     ]
   }

net/net.gyp

@@ -569,7 +569,6 @@
         }],
         ['chromeos==1', {
           'sources!': [
-            'base/network_change_notifier_linux_unittest.cc',
             'proxy/proxy_config_service_linux_unittest.cc',
           ],
         }],