MediaStream API: Patch #3 of implementing navigator.getMediaDevices

Adds idl files and tests.

Initial patch (1) by tommyw@ from https://codereview.chromium.org/145583015/

BUG=338511

Review URL: https://codereview.chromium.org/286673003

git-svn-id: svn://svn.chromium.org/blink/trunk@175469 bbb929c8-8fbe-4397-9dbb-9b2b20218538