Fix event passing to overlay scrollbars when over a plugin

There are two fixes here:
1) Previously frame scrollbars never had a chance to handle an event if an element or event handler swallowed the event. So, for example, if a mousedown event listener was added to the window and the handler called preventDefault(), then the frame scrollbar would never receive mouse clicks. This change always gives frame scrollbars an opportunity to handle the event. This would also affect elements (such as plugins) that lie underneath overlay scrollbars and swallowed events.

2) Previously if an overlay scrollbar was above a plugin, clicking on the scrollbar could trigger mouse capture on the plugin, which would interfere with the scrollbar receiving events. This change prevents mouse capture on a plugin from starting if the event was initiated above a scrollbar.

BUG=369898,358248

Review URL: https://codereview.chromium.org/296003011

git-svn-id: svn://svn.chromium.org/blink/trunk@175857 bbb929c8-8fbe-4397-9dbb-9b2b20218538