Add null check in RenderWidgetHostViewChildFrame::GetNativeView()

Other functions in the class do similar null checks. See bug report for
crashes on frame_connector_->GetParentRenderWidgetHostView() call.

Bug: 1041922
Change-Id: Ifbd3023aa24bd307841bc97dbc03ed444a8bc85d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083739Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746461}

content/browser/renderer_host/render_widget_host_view_child_frame.cc

@@ -307,6 +307,9 @@ void RenderWidgetHostViewChildFrame::SetInsets(const gfx::Insets& insets) {
 }
 
 gfx::NativeView RenderWidgetHostViewChildFrame::GetNativeView() {
+  if (!frame_connector_)
+    return nullptr;
+
   RenderWidgetHostView* parent_view =
       frame_connector_->GetParentRenderWidgetHostView();
   return parent_view ? parent_view->GetNativeView() : nullptr;