Don't use the wheel event handler count to track if a page has horizontal scrollbars

Don't use the wheel event handler count to track if a page has horizontal scrollbars
https://bugs.webkit.org/show_bug.cgi?id=78192

Reviewed by Andreas Kling.

Source/WebCore:

Stop calling Document::didAddWheelEventHandler and Document::didRemoveWheelEventHandler when
adding and removing scrollbars.

* page/FrameView.cpp:
* page/FrameView.h:
(FrameView):
* rendering/RenderLayer.cpp:
* rendering/RenderLayer.h:

Source/WebKit2:

Prior to this change, we were incrementing and decrementing the wheel event handler count
whenever a scrollable area gained or lost a horizontal scrollbar, so we could use the count
to determine if Safari can handle horizontal wheel events directly or whether they have to be sent
to the web process first.

What this meant was that whenever a page had horizontal scrollbars we'd have to send all scroll wheel events
to the main thread instead of the scrolling thread, regardless of whether there were any wheel event handlers.

After this change, we traverse the tree of scrollable areas after every layout and check if any of them
have a horizontal scrollbar. (We still also check if there are wheel event handlers).

If traversing the tree after every layout is deemed to slow we can go back to caching the number of horizontal scrollbars
in a page, but the number of subframes in a page is usually very small and the number of other scrollable areas is even smaller.

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::WebPageProxy):
(WebKit::WebPageProxy::willHandleHorizontalScrollEvents):
* UIProcess/WebPageProxy.h:
(WebKit::WebPageProxy::setCanShortCircuitHorizontalWheelEvents):
(WebPageProxy):
* UIProcess/WebPageProxy.messages.in:
* WebProcess/WebCoreSupport/WebChromeClient.cpp:
(WebKit::WebChromeClient::numWheelEventHandlersChanged):
* WebProcess/WebCoreSupport/WebFrameLoaderClient.cpp:
(WebKit::WebFrameLoaderClient::dispatchDidLayout):
* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::WebPage):
(WebKit::WebPage::numWheelEventHandlersChanged):
(WebKit):
(WebKit::hasEnabledHorizontalScrollbar):
(WebKit::pageContainsAnyHorizontalScrollbars):
(WebKit::WebPage::recomputeShortCircuitHorizontalWheelEventsState):
* WebProcess/WebPage/WebPage.h:
(WebPage):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107168 268f45cc-cd09-0410-ab3c-d52691b4dbfc