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This code is currently shared between file_manager app and
webstore_widget app. The latter is not longer used and will be removed.
Given that file manager will becaume the only user of the widget
currently under components/chrome_apps/webstore_widget/cws_widget, move
this to ui/file_manager.
(This breaks webstore widget app, but as stated before, the app is no
longer used)

BUG=839992
TEST=Verify that installing new file system service in file manager
     works.

Change-Id: Ibe0778b2bb8ef075765ce864835751414baa8841
Reviewed-on: https://chromium-review.googlesource.com/c/1288076Reviewed-by: Stuart Langley <slangley@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601052}