Remove redundant [WillBeGarbageCollected] uses on derived EventTargets.

[WillBeGarbageCollected] is already inherited from the parent EventTarget.

R=haraken
BUG=

Review URL: https://codereview.chromium.org/653263006

git-svn-id: svn://svn.chromium.org/blink/trunk@183800 bbb929c8-8fbe-4397-9dbb-9b2b20218538