Merge branch 'stable-3.0' into stable-3.1

* stable-3.0:
  ContentTypeUtil: Add singleton scope
  Upgrade bazlets to latest stable-3.0 to build with 3.0.15 API
  Upgrade bazlets to latest stable-2.16 to build with 2.16.26 API
  Upgrade bazlets to latest stable-2.16 to build with 2.16.23 API

Change-Id: I95b8d5046ba5eaebc4b3213ca6d26e06861a7943