PaygenStage: Switch to using WaitUntilReady.

Now that we have a nice interface to keep a stage from running, until
prerequisites are ready... use it!

BUG=None
TEST=cbuildbot/run_tests

Change-Id: If020028d88381622be8ae378d8e2212b7bf96e1e
Reviewed-on: https://chromium-review.googlesource.com/339481
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
2 files changed