Add [WillBeGarbageCollected] to Event.idl

- 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); }

- This CL adds a trace() metho to SpeechRecognitionError, because I found that it inherits Event.

BUG=340522

Review URL: https://codereview.chromium.org/182763002

git-svn-id: svn://svn.chromium.org/blink/trunk@168100 bbb929c8-8fbe-4397-9dbb-9b2b20218538