Rename legacyEncryptedMediaEnabled to prefixedEncryptedMediaEnabled.

The new name better reflects what this feature is for.

This the the Blink side of the change. The old names will be dropped after the Chromium side is updated as well.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/24294004

git-svn-id: svn://svn.chromium.org/blink/trunk@158246 bbb929c8-8fbe-4397-9dbb-9b2b20218538