Avoid DCHECK when the AXSelection base and/or extent are invalid.

Bug: 1013183
Change-Id: I15fc977a86773fca393a6ebd9ab09a1d52ab2934
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1853125
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704780}

third_party/blink/renderer/modules/accessibility/ax_selection.cc

@@ -156,6 +156,9 @@ AXSelection AXSelection::FromCurrentSelection(
       *ax_text_control, static_cast<int>(text_control.selectionEnd()),
       extent_affinity);
 
+  if (!ax_base.IsValid() || !ax_extent.IsValid())
+    return {};
+
   AXSelection::Builder selection_builder;
   selection_builder.SetBase(ax_base).SetExtent(ax_extent);
   return selection_builder.Build();