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Previously, this was a private class in app_list_folder_item.cc. It will
soon be required from multiple locations (to generate folder icons), so
it deserves its own file.

Also moved methods/constants relating to the tiny icon size/position
into static methods of this class (as needed by various clients).

BUG=425444

Review URL: https://codereview.chromium.org/657793004

Cr-Commit-Position: refs/heads/master@{#301028}