Rename 'prefixed' to 'legacy' in EventTarget::fireEventListeners()

Rename 'prefixed' to 'legacy' in EventTarget::fireEventListeners() now that
legacy but unprefixed events such 'mousewheel' are also handled.

R=arv

Review URL: https://chromiumcodereview.appspot.com/22877036

git-svn-id: svn://svn.chromium.org/blink/trunk@156484 bbb929c8-8fbe-4397-9dbb-9b2b20218538