Add video_loopback to gn.

webrtc/webrtc_tests.gypi

Index: webrtc/webrtc_tests.gypi
diff --git a/webrtc/webrtc_tests.gypi b/webrtc/webrtc_tests.gypi
index a05b2141cc84a6f99d2a02d61a3632033686e3e5..6a7936cc5f9f83d6befbf298bfe66eea98cbb1fe 100644
--- a/webrtc/webrtc_tests.gypi
+++ b/webrtc/webrtc_tests.gypi
@@ -277,14 +277,16 @@
       'target_name': 'video_loopback',
       'type': 'executable',
       'sources': [
-        'test/mac/run_test.mm',
-        'test/run_test.cc',
         'test/run_test.h',
         'video/video_loopback.cc',
       ],
       'conditions': [
         ['OS=="mac"', {
-          'sources!': [
+          'sources': [
+            'test/mac/run_test.mm',
+          ],
+        }, {
+          'sources': [
             'test/run_test.cc',
           ],
         }],
@@ -293,10 +295,10 @@
         'video_quality_test',
         '<(DEPTH)/testing/gtest.gyp:gtest',
         '<(DEPTH)/third_party/gflags/gflags.gyp:gflags',
+        '<(webrtc_root)/system_wrappers/system_wrappers.gyp:metrics_default',
+        'test/test.gyp:field_trial',
         'test/test.gyp:test_common',
-        'test/test.gyp:test_main',
         'test/test.gyp:test_renderer',
-        'webrtc',
       ],
     },
     {