Use an original event target instead of a target in determining dispatch behavior.

The result should be equivalent. The motivation of this change is that we can delay calculating adjusted targets, which makes the pipelines of calculating event path simpler in a follow-up patch.

BUG=305924

Review URL: https://codereview.chromium.org/26813002

git-svn-id: svn://svn.chromium.org/blink/trunk@159361 bbb929c8-8fbe-4397-9dbb-9b2b20218538