Move build tasks to MacOS 10.12

BUG=skia:

Change-Id: I5317dde5262ff6e880fa54a660f09205c616f1a4
Reviewed-on: https://skia-review.googlesource.com/9898
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Stephan Altmueller <stephana@google.com>
2 files changed