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Some devices like VM or chromeboxes do not have built-in camera at all,
we should not treat it as an error.

BUG=b:77833131,b:64996728,chromium:832816,chromium:852302
TEST=external camera works on teemo (a chromebox)

Change-Id: I2a19afa72ab9e0828f4c65a7761cd2b54d1ecfdd
Reviewed-on: https://chromium-review.googlesource.com/1136451Reviewed-by: Ricky Liang <jcliang@chromium.org>
Commit-Queue: Shik Chen <shik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574884}