Fix a crash related to null BrowserPluginGuest on Mac

content/browser/frame_host/render_widget_host_view_guest.cc

Index: content/browser/frame_host/render_widget_host_view_guest.cc
diff --git a/content/browser/frame_host/render_widget_host_view_guest.cc b/content/browser/frame_host/render_widget_host_view_guest.cc
index b14a741cd2243b1cdd955777019c5b69c83d04f8..4c649d349fa14e48e8081d8a8d905c9416fdb86f 100644
--- a/content/browser/frame_host/render_widget_host_view_guest.cc
+++ b/content/browser/frame_host/render_widget_host_view_guest.cc
@@ -544,8 +544,9 @@ void RenderWidgetHostViewGuest::UnlockCompositingSurface() {
 
 RenderWidgetHostViewBase*
 RenderWidgetHostViewGuest::GetOwnerRenderWidgetHostView() const {
-  return static_cast<RenderWidgetHostViewBase*>(
-      guest_->GetOwnerRenderWidgetHostView());
+  return guest_ ? static_cast<RenderWidgetHostViewBase*>(
+                      guest_->GetOwnerRenderWidgetHostView())
+                : nullptr;
 }
 
 // TODO(wjmaclean): When we remove BrowserPlugin, delete this code.