Fire accessible event on web area object when scrolling

Scroll events for the main document were accidentally removed in bug
371039 - this is a better way to restore them by firing them on the
web area, rather than the ScrollView/FrameView.

This brings back the "scroll" sounds on Android, and also makes Android
update the position of accessibility focus after scrolling.

BUG=371039, 374316, 414291

Review URL: https://codereview.chromium.org/662913002

git-svn-id: svn://svn.chromium.org/blink/trunk@184002 bbb929c8-8fbe-4397-9dbb-9b2b20218538