Rethink DM output

Rethink DM output

Instead of printing out each task as it finishes, or as it starts and as it finishes, periodically print out what we're up to.

Example output:
https://build.chromium.org/p/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-Shared-Trybot/builds/6436/steps/dm/logs/stdio

BUG=skia:4978
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1730943003

Review URL: https://codereview.chromium.org/1730943003