Remove cr.Event in favor of Event

DOM has improved a lot since we first started. We can now use the native
Event constructor.

BUG=None

Review URL: https://codereview.chromium.org/26891004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228259 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228259