Merge branch 'stable-2.15'

* stable-2.15:
  Upgrade bazlets to latest stable-2.15 to build with 2.15.8 API
  Upgrade bazlets to latest stable-2.14 to build with 2.14.18 API
  Bind RepoSizeEventCreator to UsageDataEventCreator
  Bazel: Include eclipse-out directory in .bazelignore
  Add explanatory comment to empty BUILD file(s)
  Update mockito to 2.23.4

Change-Id: Id0400ac77d6c5f153bbc71956514d7503fef902b