Remove *LegacyEncryptedMedia* from WebRuntimeFeatures.

This was replaced by *PrefixedEncryptedMedia*. See: https://codereview.chromium.org/24294004/

BUG=none

Review URL: https://chromiumcodereview.appspot.com/24813004

git-svn-id: svn://svn.chromium.org/blink/trunk@158405 bbb929c8-8fbe-4397-9dbb-9b2b20218538