Focus request details tab when revealed

This CL fixes an accessibility issue in the network panel where the
details pane appears without taking focus or otherwise revealing its
existence to screen reader users. The fix is to focus the details pane
when it's revealed by invoking an item in the datagrid.

Video: https://i.imgur.com/jJDhT2k.mp4

Bug: 963183
Change-Id: Ib90da5bacec59748e19b1d3788c884c8378dd24f
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2189470
Commit-Queue: Jack Lynch <[email protected]>
Reviewed-by: Robert Paveza <[email protected]>
diff --git a/front_end/network/RequestHeadersView.js b/front_end/network/RequestHeadersView.js
index 7bb5c14..a551bca 100644
--- a/front_end/network/RequestHeadersView.js
+++ b/front_end/network/RequestHeadersView.js
@@ -69,6 +69,7 @@
     const generalCategory = new Category(root, 'general', Common.UIString.UIString('General'));
     generalCategory.hidden = false;
     this._root = generalCategory;
+    this.setDefaultFocusedElement(this._root.listItemElement);
     this._urlItem = generalCategory.createLeaf();
     this._requestMethodItem = generalCategory.createLeaf();
     this._statusCodeItem = generalCategory.createLeaf();