Avoid copy on move-conversion from RepeatingCallback to OnceCallback

CallbackBase didn't have the move-conversion constructor and move-conversion
assignment. That causes an extra copy on the conversion from rvalue
RepeatingCallback to OnceCallback.

Review-Url: https://codereview.chromium.org/2740703002
Cr-Commit-Position: refs/heads/master@{#455404}
2 files changed