Google3 <functional> has a global "using std::plus"...

... so we can't use the name plus().  -_-

TBR=tomhudson@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2183453003

Review-Url: https://codereview.chromium.org/2183453003
1 file changed