formalize host debugging

Pathops writes files, anticipating upcoming crashes,
and verifies the results against regions.

Formalize these debugging methods so that they
are more easily triggered by hosts outside of
skia unit tests.

TBR=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2441763003

Review-Url: https://codereview.chromium.org/2441763003
13 files changed