Add cpu dimension for Build tasks.

This has no functional impact, but it makes it easier to see how much
bot capacity we are using.

Change-Id: I4a882c6dec438bedff821dc5bf984adac8c0cf45
Reviewed-on: https://skia-review.googlesource.com/51540
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
2 files changed