Restructure V4L2VDA to introduce v4l2 devices

This change restructures the V4L2 VDA by
introducing a V4L2Device abstraction to
facilitate use of V4L2VDA on multiple platforms

BUG=chromium-os-partner:23082
TEST=Run video playback

Review URL: https://codereview.chromium.org/136603002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247090 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247090