Revert "[Nearby] Add GetShareType to Attachment."

This reverts commit b8cd7ecee8785980ceb142ab9e15ef863a96c4af.

Reason for revert: Seems to have broken NearbyPreviewTest.All on linux-chromeos-dbg, see crbug.com/1156833

Original change's description:
> [Nearby] Add GetShareType to Attachment.
>
> Moves the logic for determining the ShareType into the *Attachment
> classes. This cleans up some static casts and also allows this logic to
> be reused when generating the preview for the receive dialog page,
> coming in a subsequent CL.
>
> To avoid a dependency cycle I've moved the ShareType mojo struct
> definition into its own build target, which required importing new mojo
> gen files in a number of places.
>
> Fixed: 1144942
> Change-Id: I7fe151470c9067a918363152d2e0520a08dc0298
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2582523
> Commit-Queue: Michael Hansen <hansenmichael@google.com>
> Reviewed-by: Alex Gough <ajgo@chromium.org>
> Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
> Reviewed-by: James Vecore <vecore@google.com>
> Cr-Commit-Position: refs/heads/master@{#836953}

TBR=khorimoto@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com,ajgo@chromium.org,vecore@google.com,hansenmichael@google.com

Change-Id: I1c7f146adaf4bf76a032a1fe7a469fcd6a05ee65
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2592991
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837024}