[sequencemanager] Move EnqueueOrder out of the internal namespace.

The goal is to allow EnqueueOrder to be used to identify tasks
outside of SequenceManager code, for example in task
ready/started/completed callbacks in
https://chromium-review.googlesource.com/c/chromium/src/+/1689339.

It also makes sense to move EnqueueOrder out of the internal
namespace since it was already exposed on non-public methods of
the non-internal base::sequence_manager::Task class.

Bug: 984057

Change-Id: I6ddd83c55d3a21350b6ff12fbc02a68800b653a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1700266
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677423}
12 files changed