Make the ability to view draft changes a grantable permission

Currently only the change owner and explicitly added reviewers
are able to see draft changes.

This change makes the ability to view draft changes a grantable
permission.  Users having this permission will be able to view
draft changes even if they are not the owner or already added
as a reviewer.

A use case for this would be a non-interactive user such as an
automated verification bot that needs to react to all changes,
including drafts.

Bug: issue 1585
Change-Id: Ia57f000d1ac075e61edb604682e85d2d135c50f2

Documentation/access-control.txt

diff --git a/Documentation/access-control.txt b/Documentation/access-control.txt
index efd1f01..2c22c45 100644
--- a/Documentation/access-control.txt
+++ b/Documentation/access-control.txt
@@ -759,6 +759,18 @@
 See above for details on each category.
 
 
+[[category_view_drafts]]
+View Drafts
+~~~~~~~~~~~
+
+This category permits users to view draft changes uploaded by other
+users.
+
+The change owner and any explicitly added reviewers can always see
+draft changes (even without having the `View Drafts` access right
+assigned).
+
+
 [[category_makeoneup]]
 Your Category Here
 ~~~~~~~~~~~~~~~~~~