[PSON] Avoid tearing down the drawing area when suspending a WebPage due to process...

[PSON] Avoid tearing down the drawing area when suspending a WebPage due to process-swap
https://bugs.webkit.org/show_bug.cgi?id=190879

Reviewed by Antti Koivisto.

Avoid tearing down the drawing area when suspending a WebPage due to process-swap. We really only need to reset
the drawing area upon resuming the WebPage. There is no strict need to destroy the drawing area on suspension
and this has caused various crashes because code usually assumes we always have a drawing area.

This patch also drops various drawing area null checks that were added to address PSON crashes.

* UIProcess/SuspendedPageProxy.cpp:
(WebKit::SuspendedPageProxy::tearDownDrawingAreaInWebProcess): Deleted.
* UIProcess/SuspendedPageProxy.h:
* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::suspendCurrentPageIfPossible):
(WebKit::WebPageProxy::receivedNavigationPolicyDecision):
(WebKit::WebPageProxy::didCompletePageTransition):
(WebKit::WebPageProxy::enterAcceleratedCompositingMode):
* UIProcess/WebPageProxy.h:
* WebProcess/WebCoreSupport/WebChromeClient.cpp:
(WebKit::WebChromeClient::invalidateContentsAndRootView):
(WebKit::WebChromeClient::invalidateContentsForSlowScroll):
(WebKit::WebChromeClient::contentsSizeChanged const):
* WebProcess/WebCoreSupport/WebFrameLoaderClient.cpp:
(WebKit::WebFrameLoaderClient::transitionToCommittedForNewPage):
* WebProcess/WebPage/RemoteLayerTree/RemoteScrollingCoordinator.mm:
(WebKit::RemoteScrollingCoordinator::scheduleTreeStateCommit):
* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::reinitializeWebPage):
(WebKit::WebPage::exitAcceleratedCompositingMode):
(WebKit::WebPage::setIsSuspended):
(WebKit::WebPage::tearDownDrawingAreaForSuspend): Deleted.
* WebProcess/WebPage/WebPage.h:
* WebProcess/WebPage/WebPage.messages.in:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@237467 268f45cc-cd09-0410-ab3c-d52691b4dbfc