Fixed crash in RenderViewContextMenuViews::Show on Windows

1. Added null check for submenu Host in RenderViewContextMenuViews::
Show() to avoid crash.

Bug: 1054962
Change-Id: Iff52355a89c64038dd3f7828a638b804563d4ae2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2070960Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Xiao Yang <yanxiao@google.com>
Cr-Commit-Position: refs/heads/master@{#744794}

chrome/browser/ui/views/renderer_context_menu/render_view_context_menu_views.cc

@@ -303,12 +303,14 @@ void RenderViewContextMenuViews::Show() {
   RunMenuAt(top_level_widget, screen_point, params().source_type);
 
   for (auto& observer : observers_) {
-    observer.OnContextMenuShown(
-        params_, static_cast<ToolkitDelegateViews*>(toolkit_delegate())
-                     ->menu_view()
-                     ->GetSubmenu()
-                     ->host()
-                     ->GetWindowBoundsInScreen());
+    auto* submenu_host = static_cast<ToolkitDelegateViews*>(toolkit_delegate())
+                             ->menu_view()
+                             ->GetSubmenu()
+                             ->host();
+    if (submenu_host) {
+      observer.OnContextMenuShown(params_,
+                                  submenu_host->GetWindowBoundsInScreen());
+    }
   }
 }