[WK2] Push wheel events if there are too many in queue

It is possible that a whole bunch of messages added to the message queue, or a series of long-running messages, cause unresponsiveness. The reason for this is that we have a scroll event waiting for acknowledgment from the web process before it sends the next event. And in the time between the user has scrolled, causing a large backlog of scroll events to be held in the UI process.

<rdar://problem/11390790>

Created an attachment (id=140435) [details]
Patch

(From update of attachment 140435 [details])
View in context: https://bugs.webkit.org/attachment.cgi?id=140435&action=review

> Source/WebKit2/UIProcess/WebPageProxy.h:1031
> +    class CoalescedWheelEvent : public RefCounted<CoalescedWheelEvent> {
> +    public:
> +        static PassRefPtr<CoalescedWheelEvent> create();
> +        CoalescedWheelEvent();
> +        virtual ~CoalescedWheelEvent();
> +        Vector<NativeWebWheelEvent> events;
> +    };

Is it necessary for this whole class to be public?  It seems like at the very least, the constructor should be private.  I'm also not clear on why this needs to be RefCounted.

Created an attachment (id=140475) [details]
Patch

(From update of attachment 140475 [details])
Originally had some problems using just the Vector but figured it out! Got rid of the internal class.

Committed r116466: <http://trac.webkit.org/changeset/116466>