Update PixelXL bots to O developer preview 3

Bug: skia:6724
Change-Id: I85a2ceed9a2dd662ddd51bfbee6cf185a3fa0eb0
Reviewed-on: https://skia-review.googlesource.com/19810
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed