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This will be required for supplying thumbnails to download OfflineContentProvider.
Since the invoking code lives in chrome/browser, we are moving the thumbnail
generation part to platform-independent code as well

Bug: 855330
Change-Id: Ic397e50faa511c83f1241d4c961c5e888ea8d272
Reviewed-on: https://chromium-review.googlesource.com/1111652Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570252}