Remove unused members from WorkerScriptController.

Also clarify some comments (I'm not sure why this needs this stuff
callable from any thread anyway) and organize the members by user.

BUG=

Review URL: https://codereview.chromium.org/553553002

git-svn-id: svn://svn.chromium.org/blink/trunk@181513 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Crashed at ImageEventListener::cast & ConditionEventListener::cast.
Add null checking in EventTarget::removeEventListener,
because we shouldn't be passing in a null listener reference to the ==()operator.

BUG=377425
R=sigbjornf@opera.com

Review URL: https://codereview.chromium.org/539453002

git-svn-id: svn://svn.chromium.org/blink/trunk@181514 bbb929c8-8fbe-4397-9dbb-9b2b20218538