Following discussion on https://codereview.chromium.org/1803423002/ and https://codereview.chromium.org/1806033002/diff/1/ui/gfx/BUILD.gn, bundle_data does not supports directories (well, it does if you omit the trailing "/").

Instead this should be supported, if bundle_data is testonly (the reasoning can be found in https://codereview.chromium.org/1611363003, TL;DR: we do not want to list all files for test data but for other targets we want to list all of them).