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This is the initial implementation of a prefetch view item for the new
download home UI.  This change includes the following:
- Builds out the ViewHolder that can handle thumbnails (automatically
processes it and pushes it to the view).
- Builds out the ViewHolder that can handle a 3-dot menu (automatically
handles the callback information).
- Adds the initial XML for the prefetched item.

BUG=849584

Change-Id: I965df4b6bdffc73f084f2f18c15588488b4beb0f
Reviewed-on: https://chromium-review.googlesource.com/1087319
Commit-Queue: David Trainor <dtrainor@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565079}