Fix crash due to RemoteLayerTreeDisplayRefreshMonitor outliving RemoteLayerTreeDrawin...

Fix crash due to RemoteLayerTreeDisplayRefreshMonitor outliving RemoteLayerTreeDrawingArea
https://bugs.webkit.org/show_bug.cgi?id=147124
<rdar://problem/21582858>

Reviewed by Simon Fraser.

Refactors RemoteLayerTreeDisplayRefreshMonitor to use a weak pointer rather than a reference
to its RemoteLayerTreeDrawingArea, since the drawing area may be deallocated before the monitor
in some rare cases. This rarely caused pages using requestAnimationFrame to crash on iOS. However,
this should not be the case: logically, a RemoteLayerTreeDrawingArea should always outlive its
refresh monitors. Refer to https://bugs.webkit.org/show_bug.cgi?id=147128 for more details.

* WebProcess/WebPage/Cocoa/RemoteLayerTreeDisplayRefreshMonitor.h:
* WebProcess/WebPage/Cocoa/RemoteLayerTreeDisplayRefreshMonitor.mm:
(WebKit::RemoteLayerTreeDisplayRefreshMonitor::RemoteLayerTreeDisplayRefreshMonitor):
(WebKit::RemoteLayerTreeDisplayRefreshMonitor::~RemoteLayerTreeDisplayRefreshMonitor): On destruction, checks
    first to see whether or not the drawing area has been deallocated before telling it to update its monitors.
(WebKit::RemoteLayerTreeDisplayRefreshMonitor::requestRefreshCallback):
* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.h:
(WebKit::RemoteLayerTreeDrawingArea::createWeakPtr): Creates and returns a new weak pointer to itself.
* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.mm:
(WebKit::RemoteLayerTreeDrawingArea::RemoteLayerTreeDrawingArea):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@187050 268f45cc-cd09-0410-ab3c-d52691b4dbfc