Merge branch 'stable-2.15' into stable-2.16

* stable-2.15:
  FileBasedWebSessionCacheCleaner: Make CleanupTask a static inner class
  Upgrade bazlets to latest stable-2.14 to build with 2.14.20 API

Change-Id: I050849b3edecf2ad40ca1538d4e3bda62a593543