AXSelection: Don't explicitly check if the endpoints are detached

This is a follow-up change to http://crrev.com/c/2346329 to address a
missed comment.

AXPosition::IsValid will check if the positions are valid, so update
AXSelection to account for any triggered DOM mutations, and then call
AXSelection::IsValid, which will check the validity of the AXPosition
instances associated with the selection.

Bug: 1113650

AX-Relnotes: N/A
Change-Id: Ic7d72111b2172784bbd199b6fe7767b5badefa43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2396156
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805405}

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

@@ -332,7 +332,8 @@ const SelectionInDOMTree AXSelection::AsSelection(
 
 void AXSelection::UpdateSelectionIfNecessary() {
   Document* document = base_.ContainerObject()->GetDocument();
-  DCHECK(document);
+  if (!document)
+    return;
 
   LocalFrameView* view = document->View();
   if (!view || !view->LayoutPending())
@@ -397,13 +398,9 @@ bool AXSelection::Select(const AXSelectionBehavior selection_behavior) {
     return false;
   }
 
-  // If the anchor or focus is removed during the "selectstart" event, do
-  // not proceed.
-  if (base_.ContainerObject()->IsDetached() ||
-      extent_.ContainerObject()->IsDetached())
-    return false;
-
   UpdateSelectionIfNecessary();
+  if (!IsValid())
+    return false;
 
   // Dispatching the "selectstart" event could potentially change the document
   // associated with the current frame.