Prevent ctrl+wheel from scrolling in more places

Chromium now sends ctrl+wheel events to webkit first before using them for zooming. Rather than prevent scrolling for these events in EventHandler::handleWheel, push that check a couple levels deeper into ScrollableArea::handleWheelEvent.  In addition to EventHandler, this code is also called by WebPluginScrollbarImpl and so will fix some additional cases.

This is mostly already tested by the fast/event/wheelevent-ctrl.html test I added in http://crrev.com/183403002.  I've extended that test to cover document and div scrolling cases and ensured it fails reliably if either the ScrollableArea or EventHandler::defaultWheelEventHandler check is removed. 

BUG=352474, 111059

Review URL: https://codereview.chromium.org/212913002

git-svn-id: svn://svn.chromium.org/blink/trunk@170238 bbb929c8-8fbe-4397-9dbb-9b2b20218538