Reland: Remove "needkey" event and add its 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.

TBR=jochen@chromium.org,ddorwin@chromium.org
BUG=224786
(cherry picked from commit ad09c3ed1914691ee4a089cfe95988892ed42407)

Review URL: https://codereview.chromium.org/655603003

git-svn-id: svn://svn.chromium.org/blink/trunk@183628 bbb929c8-8fbe-4397-9dbb-9b2b20218538