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
1 file changed