Optionally run RAW images serially

RAW images use a lot of memory. Add a new FLAG to run one at a time so
we have less risk of running out of memory.

Isolate RAW images to their own thread on particular devices where our
images cause OOM errors.

Locally, this drops the max memory use from 3945 MB to 1664 MB (running only --image --images <RAW images we test>)

BUG=skia:4912
BUG=skia:4878
BUG=b/27035849
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1681553003

Review URL: https://codereview.chromium.org/1681553003
4 files changed