Oilpan: Add [WillBeGarbageCollected] to Event.idl

This is a minimal change to make [WillBeGarbageCollected] in Event.idl workable.

- This CL adds [WillBeGarbageCollected] to Event.idl and makes minimal changes
to the code base to make it workable.

- This CL is almost just replacing:

  PassRefPtr<XEvent> XEvent::create() { return adoptRef(new XEvent); }

with

  PassRefPtrWillBeRawPtr<XEvent> XEvent::create() { return
adoptRefWillBeRefCountedGarbageCollected(new XEvent); }

BUG=340522
NOTRY=true

Review URL: https://codereview.chromium.org/213363006

git-svn-id: svn://svn.chromium.org/blink/trunk@170164 bbb929c8-8fbe-4397-9dbb-9b2b20218538