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This never launched and has long been abandoned.
This CL removes the code. Followups will remove all the wiring and
indirection in ThumbnailTabHelper and ThumbnailService.

Bug: 735905
Change-Id: Ic6d125ea5c5020dcf3f484487257f9a221e59868
Reviewed-on: https://chromium-review.googlesource.com/544960
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#481834}