Track event handlers on windows

This patch makes EventHandlerRegistry keep track of event handlers
registered on the window object.

TEST=fast/events/scroll-event-handler-count.html,fast/events/scroll-event-handler-reused-window.html
BUG=359566

Review URL: https://codereview.chromium.org/240343002

git-svn-id: svn://svn.chromium.org/blink/trunk@173552 bbb929c8-8fbe-4397-9dbb-9b2b20218538