Track scroll event handlers in nested documents
    
This patch moves the EventHandlerRegistry from the Document 
object to the Page so that it tracks handlers in all 
documents on the page. This makes it possible to do global 
queries for event handlers.
    
TEST=fast/events/scroll-event-handler-count.html
BUG=359566,367297,369082

Review URL: https://codereview.chromium.org/237963014

git-svn-id: svn://svn.chromium.org/blink/trunk@173101 bbb929c8-8fbe-4397-9dbb-9b2b20218538