events: Introduce EventTargeter.

EventTargeter will be used for finding the target for dispatching events in
aura and views. More details about the design at the document linked at the
bug.

This patch introduces an EventTargeter interface, with an empty default
implementation. It also adds some functions to the EventTarget interface
that will be used by the actual EventTargeter implementation.

BUG=318879
R=ben@chromium.org

Review URL: https://codereview.chromium.org/98293002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238146 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238146