Don't clobber focus in chromecast overridden GetTreeData method

Focus should only be set if the child tree is found.
Otherwise, leave the focus set by the super method alone.
This was causing the reader to lose focus on cast and
webviews embedded in the display assistant's main tree.

Bug: NONE
Test: display assistant with youtube/weather app/page
Change-Id: If82855ada00fa679806987e8f78332799d0e7015
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2339497Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795447}

chromecast/browser/ui/aura/accessibility/ax_tree_source_aura.cc

@@ -40,7 +40,6 @@ bool AXTreeSourceAura::GetTreeData(ui::AXTreeData* tree_data) const {
         aura::client::GetFocusClient(root_window);
     if (focus_client) {
       aura::Window* window = focus_client->GetFocusedWindow();
-      tree_data->focus_id = ui::AXNode::kInvalidAXID;
 
       // Search for the full screen shell surface that holds the child
       // tree for our UI. This is the node that should receive focus.