Remove the media attribute from the resource selection algorithm

Intent to Deprecate and Remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/5sWUMC_d8Tg/ZZ0Z7rfeCqUJ

The HTMLSourceElement.media IDL attribute is left dangling, but a
UseCounter will be added for it in a separate commit, with the
intention to remove it once we have the data to support it.

BUG=338197

Review URL: https://codereview.chromium.org/147933002

git-svn-id: svn://svn.chromium.org/blink/trunk@166034 bbb929c8-8fbe-4397-9dbb-9b2b20218538