Add support for RelatedEvent.

Add support for RelatedEvent which is needed to implement contextmenu.
Specifications:
http://www.whatwg.org/specs/web-apps/current-work/#relatedevent

BUG=87553

Review URL: https://codereview.chromium.org/443053002

git-svn-id: svn://svn.chromium.org/blink/trunk@179626 bbb929c8-8fbe-4397-9dbb-9b2b20218538