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This patch:
* Adds APIs to ExploreSitesService that can request either a site image
  or a category image (which is a composite of multiple site images) as
  an SkBitmap (which is the format required for Java Bitmaps).
* Adds a task to fetch one or more images from the ExploreSitesStore,
  based on the site ID or category ID.
* The task filters based on the existence of an entry in the
  site_blacklist table (which is not populated yet but will be as a
  result of user long-press.

This DOES NOT implement conversion from byte vectors to SkBitmap (left
as a TODO.)

Bug: 867488
Change-Id: Id2e6a6abe2781b790f9ce3f597a9e747eddd4715
Reviewed-on: https://chromium-review.googlesource.com/1233176Reviewed-by: Cathy Li <chili@chromium.org>
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592501}