Get content_unittests to link on the mac GN build.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 785722298f0292f1c62deae254e877dc600d9be8..7e2cc41e65a7cc2bc124f67a607059b8eef04657 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -541,7 +541,6 @@ group("both_gn_and_gyp") {
       "//components:components_unittests",  # TODO(GYP)
       "//content/test:content_browsertests",  # TODO(GYP)
       "//content/test:content_perftests",  # TODO(GYP)
-      "//content/test:content_unittests",  # TODO(GYP)
       "//device:device_unittests",  # TODO(GYP)
       "//extensions:extensions_browsertests",  # TODO(GYP)
       "//extensions:extensions_unittests",  # TODO(GYP)