Declare string16 traits’ move() correctly.
Declare string16 traits’ move() correctly.

As in upstream https://codereview.chromium.org/1020333002/,
https://crrev.com/fd697fc2963b8adccc629356fd94cbc08f3b9880.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/1028553004

bowlofstew/mini_chromium,chromium/mini_chromium,chromium/mini_chromium,Chilledheart/mini_chromium,Chilledheart/mini_chromium,bowlofstew/mini_chromium,chromium/mini_chromium,Chilledheart/mini_chromium,bowlofstew/mini_chromium