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- CupsPrintersEntryManager contains both saved and nearby printers.
- This new class handles the model manipulation of printers and calls
  on its listeners to let the views know when the models have been
  updated.
- Adds CupsPrintersEntryListBehavior to observe the change events
  from CupsPrintersEntryManager.
- Updated tests to reflect this change.

Bug: 1009659
Test: End to end manual, browsertests
Change-Id: I86f2a4d521de644a0fb4b5e3c01ef55e704642a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1830292Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: jimmy gong <jimmyxgong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705150}