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Changes:
1. Load item list with "page break" items into paged view structure
   which will be displayed to the user.
2. Add "page break" item based on the changed paged view structure after
   user operations (Add, Remove, Move an item).
3. Since model index (item view's index in view model) is no longer the
   same as item index (item's index in item list), separate them.

BUG=848917

Change-Id: I5d4c750ab87045a0e4930e549a066f24a1c1cf66
Reviewed-on: https://chromium-review.googlesource.com/1083492Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564973}