Oilpan: Remove a bunch of WillBeRefCountedGarbageCollected occurrences.

They used to be needed for EventTargets but that is no longer the case.

Review URL: https://codereview.chromium.org/491783002

git-svn-id: svn://svn.chromium.org/blink/trunk@180637 bbb929c8-8fbe-4397-9dbb-9b2b20218538