Do not set the cache to valid when the device list is empty.

In some cases, for example when the machine just wakes up from sleeping, the camera might not be ready for enumeration. This will fool Chrome that there is no device attaching to the machine, and fail all the getUserMedia call.

This patch fixes the problem by not setting the cache to be valid when the list is empty.

BUG=317673
TEST=using pixel, open the camera using, apprtc.appspot.com, put down the lip, wait until the machine gets to sleep, then wake up and use apprtc.appspot.com. The camera should work, at least after refreshing.
R=perkj@chromium.org

Review URL: https://codereview.chromium.org/66303011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234532 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234532