another layer of step nesting
another layer of step nesting

Group batches from a single kickoff() into another mid-level step:

Top-Level
    kickoff --some flags
        batch sources...
            batch (exec)
        batch other sources ...
            batch (exec)
            rerun (exec)
            rerun (exec)
        batch yet other sources ...
            batch (exec)
            rerun (exec)
    kickoff --some other flags
        ...

Big question: is it okay for the kickoff steps to td.EndStep() while its
kids are still running (or haven't even started) on other goroutines?

Change-Id: I77ad2274e35cea0151be0cca6c690eafc4f8983e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/366140
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>

aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia