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a.Then(b) will return a new callback that when Run() will
1) run |a|
2) run |b|, passing it the return value from a
3) return the result from |b|

OnceCallbacks must be destroyed when joining them together with Then(),
so the method is rvalue-qualified. This means it is used in the
same way as Run(), for example, this function posts two callbacks to
run together as a single task:

void PostTwoTasks(base::OnceClosure c1, base::OnceClosure c2) {
  PostTask(std::move(c1).Then(std::move(c2)));
}

RepeatingCallback can be joined destructively via the rvalue-qualified
overload, or non-destructively otherwise. The latter is allowed for a
RepeatingCallback because it is meant to have multiple callers and
therefore having both the original callbacks and the joined callback
pointing to the same underlying functor is not problematic.

R=chrisha@chromium.org, gab@chromium.org

Bug: 1140582
Change-Id: Ie147f01d1c8adeb5ed34e4933f211e7d247e3c6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485642
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820319}