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When permission for a webcam has not yet been granted, we still allow
enumerating the devices, but omit the name. Some Flash content can't
deal with an empty string for the webcam name. This turns the empty
string in to a space to work around the problem.

BUG=408404

Review URL: https://codereview.chromium.org/577983002

Cr-Commit-Position: refs/heads/master@{#295476}