Support macro derive(BitField) to make life easier.
BUG=None.
TEST=local build and run test.
Change-Id: I582620de250017fb7c0b601f9ad4fbcbbc2fe02a
Reviewed-on: https://chromium-review.googlesource.com/1069331
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Jingkui Wang <jkwang@google.com>
Reviewed-by: Zach Reizner <zachr@chromium.org>
