Add EventHandlerRegistry
    
This patch introduces a generic EventHandlerRegistry document supplement
for keeping track for event handlers on EventTargets.
    
The EventHandlerRegistry provides a hash map of EventTargets for scroll
events; touch and wheel events will be added later. Currently the target
set contains all event handler targets in the host document. Future
patches will add handlers from child documents.
    
Initially the registry only tracks scroll handlers, but upcoming patches
will migrate the existing code paths for tracking touch and wheel events
to use this registry instead.
    
TEST=fast/events/scroll-event-handler-count.html
BUG=347366

Review URL: https://codereview.chromium.org/206603002

git-svn-id: svn://svn.chromium.org/blink/trunk@171885 bbb929c8-8fbe-4397-9dbb-9b2b20218538