Add Win7 and Win8 bots.

Bug: skia:6517
No-Try: true
Change-Id: I02abf8aae6c356be23612f2d5442e6837919a1dc
Reviewed-on: https://skia-review.googlesource.com/13597
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
3 files changed