[Speculative revert to fix browser_tests on Blink canaries.]

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

This reverts commit ad09c3ed1914691ee4a089cfe95988892ed42407.

BUG=
TBR=sandersd

Review URL: https://codereview.chromium.org/647383002

git-svn-id: svn://svn.chromium.org/blink/trunk@183580 bbb929c8-8fbe-4397-9dbb-9b2b20218538