Move when swapping, if possible.

This change was avoided in the past because vc++ 2013 (12.0)
did not properly create default move constructors and
move assignment operators.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2454763002

TBR=reed
Verbal lgtm

Review-Url: https://codereview.chromium.org/2454763002
2 files changed