Remove getMediaDevices interface.

getMediaDevices was landed for M37 but the spec changed just at the M37 cut without a heads-up. So we are removing getMediaDevices and replacing it with MediaDevices.enumerateDevices for M38.

This CL depends on reverting/disabling test code: https://codereview.chromium.org/341123005/

BUG=388648

Review URL: https://codereview.chromium.org/357303002

git-svn-id: svn://svn.chromium.org/blink/trunk@177480 bbb929c8-8fbe-4397-9dbb-9b2b20218538