Support base::test::ScopedTaskEnvironment and content::TestBrowserThreadBundle in the same scope.

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index 6935ee3a77802e6d0c5400bbbae8b694f51b9a16..d86262e16708e2d0e29dac9a6eb767b6767203c4 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -1362,6 +1362,7 @@ test("content_unittests") {
     "../common/webplugininfo_unittest.cc",
     "../network/url_loader_unittest.cc",
     "../public/test/referrer_unittest.cc",
+    "../public/test/test_browser_thread_bundle_unittest.cc",
     "../renderer/android/disambiguation_popup_helper_unittest.cc",
     "../renderer/bmp_image_decoder_unittest.cc",
     "../renderer/categorized_worker_pool_unittest.cc",