Convert ManifestIconDownloader::IconFetchCallback to be a OnceCallback.

TBR=gogerald@chromium.org
BUG=915945

Change-Id: I3b1015db932dc8139eadbd46b154dc3395bed922
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1514279
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639696}
8 files changed