This is a preparation CL for http://crrev.com/2637843002, which replaces
the Callback parameter of TaskRunner::PostTask with OnceCallback.
This one replaces the passed-by-const-ref Callback parameter of
TaskRunner::PostTask() with pass-by-value.

With the pass-by-const-ref manner as the old code does, we can't avoid
leaving a reference to the callback object on the original thread. That
is, the callback object may be destroyed either on the target thread or
the original thread. That's problematic when a non-thread-safe object is
bound to the callback.

Pass-by-value and move() in this CL mitigate the nondeterminism: if the
caller of TaskRunner::PostTask() passes the callback object as rvalue,
TaskRunner::PostTask() leaves no reference on the original thread.
I.e. the reference is not left if the callback is passed directly from
Bind(), or passed with std::move() as below.

  task_runner->PostTask(FROM_HERE, base::Bind(&Foo));

  base::Closure cb = base::Bind(&Foo);
  task_runner->PostTask(FROM_HERE, std::move(cb));

Otherwise, if the caller passes the callback as lvalue, a reference to
the callback is left on the original thread as we do in the previous code.
I.e. a reference is left if the callback is passed from other non-temporary
variable.

  base::Closure cb = base::Bind(&Foo);
  task_runner->PostTask(FROM_HERE, cb);

This is less controversial part of http://crrev.com/2637843002. This CL
is mainly to land it incrementally.

TBR=shrike@chromium.org
BUG=704027
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2726523002
Cr-Commit-Position: refs/heads/master@{#460288}