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This is useful for the extensions API, where it's helpful to have something like:

    tab_groups::TabGroupId group = tab_groups::TabGroupId::CreateEmpty();
    GetGroupFromId(id, &group);  // Populates group by reference, with an existing value.
    if (!group.is_empty()) {
      // Logic handling the group
    }

Having CreateEmpty() and is_empty() helps make this pattern intentional without accidentally creating empty groups and breaking assumptions in the tabstrip.

Change-Id: Ibb217edfd8ba599023488ea3d1a33277b49d8284
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415573Reviewed-by: Charlene Yan <cyan@chromium.org>
Commit-Queue: Connie Wan <connily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808028}