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* Move CupsWrapper to separate class
* Move heavy tasks to inner CupsWrapper::Backend class

Bug: 996785
Change-Id: I481e0df807017ee7e1221473baacf0d2f1a52d8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904170
Commit-Queue: Nikita Podguzov <nikitapodguzov@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714459}