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After migrating app_list from Chrome to Ash, we'll need to pass
StringOrdinal objects between processes for setting app list item
positions, e.g. AppListModel::SetItemPosition.

This commit creates a mojo struct for StringOrdinal so that
//ash/public/interfaces:interfaces_internal can depend on it.

Bug: 733662
Change-Id: I96bbd81a82a53462aa0eb2cdcf9e327045a70051
Reviewed-on: https://chromium-review.googlesource.com/780696Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Pavel Yatsuk <pavely@chromium.org>
Commit-Queue: Jiaquan He <hejq@google.com>
Cr-Commit-Position: refs/heads/master@{#521386}