dm: require tmpDir, reasonable defaults

Change-Id: I9d84ce1ebbe417160a29ca2221b1df04901238e3
Reviewed-on: https://skia-review.googlesource.com/83541
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
4 files changed