Create an event path in an event only when the event is used in a DOM tree.

IndexedDBEventDispatcher doesn't use Event::m_eventPath at all. This dispatcher calculates an *event path* by themselves.
Therefore, we should create an event path in an event only when the event is used in a DOM tree.

BUG=328410

Review URL: https://codereview.chromium.org/116363005

git-svn-id: svn://svn.chromium.org/blink/trunk@164145 bbb929c8-8fbe-4397-9dbb-9b2b20218538