GN: gfx_unittests links on Mac.

Tested on Mac with the following command lines:
$ gn gen out-gn/Release --args='is_debug=false is_component_build=false symbol_level=1'
$ ninja -C out-gn/Release gfx_unittests

BUG=431177
TEST=see above
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1287753003

Cr-Commit-Position: refs/heads/master@{#343070}

diff --git a/BUILD.gn b/BUILD.gn
index 6bde7c0..3f5ab117 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -535,6 +535,7 @@
       "//third_party/qcms",
       "//third_party/smhasher:murmurhash3",
       "//third_party/webrtc/system_wrappers",
+      "//ui/gfx:gfx_unittests",
       "//ui/native_theme",
       "//ui/snapshot",
       "//ui/surface",
@@ -562,7 +563,6 @@
       "//third_party/WebKit/Source/platform:platform_unittests",
       "//third_party/WebKit/Source/web:webkit_unit_tests",
       "//ui/app_list:app_list_unittests",  # TODO(GYP)
-      "//ui/gfx:gfx_unittests",  # TODO(GYP)
     ]
   }