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This CL refactors sharesheet_service_delegate to be identified
by its native_window. This is so that we can ensure only one
sharesheet for any one native_window at any time.

Bug: 1142706
Change-Id: I7892bb5b95f6ab4fa17c772e173084b11e3ab69f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2563010Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Melissa Zhang <melzhang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833098}