apps::SavedFilesService provides apps with a LRU queue of "retained"
file entries they are allowed access to. See chrome.fileSystem API docs:
https://developer.chrome.com/apps/fileSystem#method-retainEntry

Chrome extension APIs can already use the service in //apps, but APIs
implemented in //extensions cannot (they can't depend on //apps). This
change enables moving chrome.fileSystem into //extensions.

This CL makes apps::SavedFilesService derive from a new interface,
extensions::SavedFilesServiceInterface. The new interface took a few
methods from the original Service.

BUG=729713
R=benwells@chromium.org

Review-Url: https://codereview.chromium.org/2960433002
Cr-Commit-Position: refs/heads/master@{#482297}