Correct a copy and paste mistake, which uses the wrong pref name for video default device.

we should prefs::kDefaultVideoCaptureDevice for video type instead of kDefaultAudioCaptureDevice.

BUG=307761
TEST=https://webrtc.googlecode.com/svn/trunk/webrtc/test/manual/single-video.html
verify the device selection.

Review URL: https://codereview.chromium.org/26122006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229904 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229904