Do not reset callback after moving.

Unlike general move semantics, when a Callback (either OnceCallback or
RepeatingCallback) is moved from, it is guaranteed to be left in a null
state.  (See, for example, [1].)  This CL removes two Reset calls from
a freshly moved-from Callback, and uses std::move instead of assignment
then Reset for a third one.

[1] https://cs.chromium.org/chromium/src/base/callback_helpers.h?l=24

Bug: 807724
Change-Id: I538408b26fc0b6af601b23a669cb5447d2f9b1fe
Reviewed-on: https://chromium-review.googlesource.com/1064019
Reviewed-by: Eric Roman <eroman@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560260}
3 files changed