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This patch adds support for app icons when used as file handlers in the
file manager. As there is no place to store the CrostiniAppIcon
objects, a IconLoadWaiter is added which manually manages it's lifetime
and deletes itself after an icon is loaded (or fails to load).

BUG=822513

Change-Id: Ia31b32806e399df8902ffbf5c953e39dbc414774
Reviewed-on: https://chromium-review.googlesource.com/1127204Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576385}