Fix RenderViewImpl::CloseWindowSoon to handle null render_widget_.

In the future, undead RenderWidgets will no logner exist. This CL updates
RenderViewImpl::CloseWindowSoon to check for this condition.

Bug: 912193
Change-Id: Icbe449794080d36368eac5c9ffe90a198709dbda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1814673
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698502}

content/renderer/render_view_impl.cc

@@ -1512,7 +1512,7 @@ void RenderViewImpl::DoDeferredClose() {
 
 void RenderViewImpl::CloseWindowSoon() {
   DCHECK(RenderThread::IsMainThread());
-  if (render_widget_->IsUndeadOrProvisional()) {
+  if (!render_widget_ || render_widget_->IsUndeadOrProvisional()) {
     // Ask the RenderViewHost with a local main frame to initiate close.  We
     // could be called from deep in Javascript.  If we ask the RenderViewHost to
     // close now, the window could be closed before the JS finishes executing,