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This callback implies that the resources are gone (because LTH
was deleted). What ends up happening is that the resources get
deleted while LTH still tries to call GetBitmap() on them.

This still needs some working invalidation mechanism for Thumbnails,
which don't have a persistent Bitmap they return from GetBitmap().

BUG=406879
NOTRY=True
TBR=dtrainor@chromium.org

Review URL: https://codereview.chromium.org/543543002

Cr-Commit-Position: refs/heads/master@{#293252}