This CL adds another flag, RepeatMode, to base::Callback to make a Callback
variant that can be run only once, and exposes it as OnceCallback.
This also adds a base:::Bind variant to create an OneOnce, and
adds provides conversions and constraints between Callbacks.

BUG=554299

Review-Url: https://codereview.chromium.org/2042223002
Cr-Commit-Position: refs/heads/master@{#418177}