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On the downloads page, we consult the system for a themed image for the
type of file downloads. Ex: zip files -> archive icon.

When a system doesn't know why type of icon to show for a specific file
type, it can either return a default or nothing. Right now, Linux
returns nothing, so we show nothing.  chrome://fileicon tries to warn
us by giving a network error (which has been ignored until now).

So, when a chrome://fileicon fails to load, show a default filetype
icon in respond.

Before & after screenshots: https://imgur.com/a/txRbfIG

R=dpapad@chromium.org
BUG=855259

Change-Id: I370fd8ea119b194ad8e1ba7e0dfad32c6cea1bbe
Reviewed-on: https://chromium-review.googlesource.com/c/1132560
Commit-Queue: Dan Beam <dbeam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614581}