[skunicode] Add temporary file for transition

Because of https://github.com/bazelbuild/bazel/issues/1431 empty
filegroups can be created but cannot generally be used. To allow an
existing filegroup to be staged to be split add a temporary empty header
which the new filegroup can contain.

Change-Id: I563f94621428fc89123e76127e163dfb2511ec66
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/737858
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
2 files changed