message: Use xenial explicitly for builder
  This CL completely migrates dawn linux builder from trusty to xenial.
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"
  + dimensions: "os:Ubuntu-16.04"
  mixins: "debug"
  mixins: "x64"
  }
  builders {
  name: "linux-clang-dbg-x86"
  - dimensions: "os:Ubuntu"
  + dimensions: "os:Ubuntu-16.04"
  mixins: "debug"
  mixins: "x86"
  }
  builders {
  name: "linux-clang-rel-x64"
  - dimensions: "os:Ubuntu"
  + dimensions: "os:Ubuntu-16.04"
  mixins: "release"
  mixins: "x64"
  }
  builders {
  name: "linux-clang-rel-x86"
  - dimensions: "os:Ubuntu"
  + dimensions: "os:Ubuntu-16.04"
  mixins: "release"
  mixins: "x86"
  }
  @@ -212,7 +212,7 @@ buckets: {
  builders {
  name: "presubmit"
  - dimensions: "os:Ubuntu"
  + dimensions: "os:Ubuntu-16.04"
  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"
  + dimensions: "os:Ubuntu-16.04"
  mixins: "debug"
  mixins: "x64"
  }
  # Actually on the CQ
  builders {
  name: "linux-clang-dbg-x86"
  - dimensions: "os:Ubuntu"
  + dimensions: "os:Ubuntu-16.04"
  mixins: "debug"
  mixins: "x86"
  }
  # Actually on the CQ
  builders {
  name: "linux-clang-rel-x64"
  - dimensions: "os:Ubuntu"
  + dimensions: "os:Ubuntu-16.04"
  mixins: "release"
  mixins: "x64"
  }
  # Actually on the CQ
  builders {
  name: "linux-clang-rel-x86"
  - dimensions: "os:Ubuntu"
  + dimensions: "os:Ubuntu-16.04"
  mixins: "release"
  mixins: "x86"
  }
  