Remove "needkey" event and add it's replacement, "encrypted".

This new event is of type MediaEncryptedEvent, which has initDataType instead of contentType (but this CL just renames one to the other, without translation), and changes the type of initData from Uint8Array to ArrayBuffer as per the WD EME spec.

Also in this CL:
  - Start renaming keyNeeded() to encrypted() by renaming the it on HTMLMediaElementEncryptedMedia.
  - Remove unused contentType argument from createWebkitNeedKeyEvent() (good thing it's unused or the contentType to initDataType change would be a lot harder).
  - Fix prefixed/encrypted-media-events.html test to test the prefixed needkey event instead of the unprefixed one.

BUG=224786

Review URL: https://codereview.chromium.org/600143005

git-svn-id: svn://svn.chromium.org/blink/trunk@183561 bbb929c8-8fbe-4397-9dbb-9b2b20218538