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This CL prevents downloads by the internal download service from being
shown in the extensions API. The motivation comes from running the
Plugin VM installer while using ChromeVox -- the download progress is
reported to the user with a guid as a name, which is a bit confusing
and clashes with updates from the installer's progress bar.

In general, it seems like these internal downloads shouldn't be exposed
to the extensions API. chrome://download-internals uses a different API
to access this data so it will continue to display this data.

Bug: 1021414
Change-Id: Idabbd4548a672fe9b773289a7809c32013dda56c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2143186Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759960}