Restructure the events target into two components:

- events_base (built by most people) - defining some basic utilities.
- events (built by users of aura/views) - the ui::Event type & friends.

Prior to this there was one target, and platforms that did not want ui::Event would exclude those files explicitly, making it hard to build those files on those platforms without breaking other components.

R=sadrul@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/57433011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233347 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233347