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The thubmail loader is already checking whether the holding space item
is a folder, but it used to return an empty bitmap in this case (relying
on the placeholder icon creation to generate the icon; which relies
purely on the file path, and may thus generate an incorrect icon -
currently it generates a generic file icon; with this change this will
be WAI).
Instead, create the correct folder icon in the holding space thumbnail
loader.

BUG=1141034

Change-Id: I7d619adc892d2595284df210cefcab44f9279f43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2552370Reviewed-by: David Black <dmblack@google.com>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829966}