Oilpan: Remove ref counting from EventTarget and all uses of Pass/RefPtr<EventTarget>.

BUG=357163

Review URL: https://codereview.chromium.org/316443004

git-svn-id: svn://svn.chromium.org/blink/trunk@175464 bbb929c8-8fbe-4397-9dbb-9b2b20218538