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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).

This is a re-land of https://crrev.com/c/1127204, but with the longer
deletion time-out removed. It turns out the longer time-out is not
needed, and it was causing LSAN to detect a leak as the objects were
not cleaned up by test completion.

BUG=822513

Change-Id: Id04730ee2ea18f6f5004624471e40bcd13e9974b
Reviewed-on: https://chromium-review.googlesource.com/1149432Reviewed-by: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577822}