A hung webpage pretends to be responsive if you scroll

A hung webpage pretends to be responsive if you scroll
https://bugs.webkit.org/show_bug.cgi?id=151518

Reviewed by Sam Weinig.

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::sendWheelEvent):
(WebKit::WebPageProxy::didReceiveEvent): Don't treat wheel events as
starting or stopping the responsiveness timer. Wheel events usually
process on the event dispatch thread, which responds even if the main
thread is hung.

Instead, send an out-of-band ping to the main thread to verify that
it is still responsive and we'll be able to paint and respond to clicks
after scrolling.

* UIProcess/WebProcessProxy.cpp:
(WebKit::WebProcessProxy::sendMainThreadPing):
(WebKit::WebProcessProxy::didReceiveMainThreadPing):
* UIProcess/WebProcessProxy.h:
* UIProcess/WebProcessProxy.messages.in: UI process support for pinging
the main thread in the web process.

* WebProcess/WebProcess.cpp:
(WebKit::WebProcess::mainThreadPing):
* WebProcess/WebProcess.h:
* WebProcess/WebProcess.messages.in: Web process support for responding
to pings.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@192702 268f45cc-cd09-0410-ab3c-d52691b4dbfc