message: Make Linux bots run on an unspecified Ubuntu version
  Previously they would only run on Ubuntu 14.04 which has been removed
  from the luci flex pool. Run on unspecified Ubuntu version so the
  tryjobs start working again.
  TBR=enga@chromium.org
diff: @@ -104,25 +104,25 @@ buckets {
  # Linux: test combinations of {clang}x{release,debug}x{x86,x64}
  builders {
  name: "linux-clang-dbg-x64"
  - dimensions: "os:Ubuntu-14.04"
  + dimensions: "os:Ubuntu"
  mixins: "debug"
  mixins: "x64"
  }
  builders {
  name: "linux-clang-dbg-x86"
  - dimensions: "os:Ubuntu-14.04"
  + dimensions: "os:Ubuntu"
  mixins: "debug"
  mixins: "x86"
  }
  builders {
  name: "linux-clang-rel-x64"
  - dimensions: "os:Ubuntu-14.04"
  + dimensions: "os:Ubuntu"
  mixins: "release"
  mixins: "x64"
  }
  builders {
  name: "linux-clang-rel-x86"
  - dimensions: "os:Ubuntu-14.04"
  + dimensions: "os:Ubuntu"
  mixins: "release"
  mixins: "x86"
  }
  @@ -212,7 +212,7 @@ buckets: {
  builders {
  name: "presubmit"
  - dimensions: "os:Ubuntu-14.04"
  + dimensions: "os:Ubuntu"
  recipe {
  name: "run_presubmit"
  properties: "repo_name:dawn"
  @@ -227,28 +227,28 @@ buckets: {
  # CI Mirror only
  builders {
  name: "linux-clang-dbg-x64"
  - dimensions: "os:Ubuntu-14.04"
  + dimensions: "os:Ubuntu"
  mixins: "debug"
  mixins: "x64"
  }
  # Actually on the CQ
  builders {
  name: "linux-clang-dbg-x86"
  - dimensions: "os:Ubuntu-14.04"
  + dimensions: "os:Ubuntu"
  mixins: "debug"
  mixins: "x86"
  }
  # Actually on the CQ
  builders {
  name: "linux-clang-rel-x64"
  - dimensions: "os:Ubuntu-14.04"
  + dimensions: "os:Ubuntu"
  mixins: "release"
  mixins: "x64"
  }
  # Actually on the CQ
  builders {
  name: "linux-clang-rel-x86"
  - dimensions: "os:Ubuntu-14.04"
  + dimensions: "os:Ubuntu"
  mixins: "release"
  mixins: "x86"
  }
  