Couple small changes to speed up Goma compiles

 - Use more threads
 - Avoid failures due to Goma compiler_proxy running after Swarming
   task terminates, e.g.
   https://chromium-swarm.appspot.com/task?id=3b309dfecdaa9b10

Change-Id: I8521ad0dbaead6f22a2d3a3401dfe0b2695742b9
Reviewed-on: https://skia-review.googlesource.com/99020
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
4 files changed