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Most test extensions manifest's have a descripton: the Files, Gallery,
Video Player, Audio Player test extension do not (how very odd). Let's
give them a description, for consistency with all test extensions.

Bug: 836254
Change-Id: I43dce817da53f9ae7a584f4b30907c3c0649ee85
Reviewed-on: https://chromium-review.googlesource.com/1156118
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579322}