Clean up forward declarations in Source/core/events

These forward references are not needed anymore. After
this patch, Source/core/events has no unneeded forward references
any more.

Review URL: https://codereview.chromium.org/656683009

git-svn-id: svn://svn.chromium.org/blink/trunk@183935 bbb929c8-8fbe-4397-9dbb-9b2b20218538