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This class adds the required infrastructure to generalize sharing across
content types.  This comes down to adding an async API to
OfflineContentProvider to request the URI (if available) to use in
generating a share intent.  This also adds the utility class to build
the intent from that set of OfflineItems and share info (URI)s.

TBR=petewil@chromium.org

BUG=850780

Change-Id: If9d5d4f01a8559bec90b7de1adb70c8aaafa9302
Reviewed-on: https://chromium-review.googlesource.com/1105125Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Mugdha Lakhani <nator@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581356}