[Switch Access] Add base implementation for isValidAndVisible

Codify the expectation that isValidAndVisible() == true implies that
location !== undefined.

AX-Relnotes: n/a.
Bug: None.
Change-Id: I7ba047b47b82086699f1c17a48263f76f6a9773e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2248349
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Auto-Submit: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779875}

chrome/browser/resources/chromeos/accessibility/switch_access/nodes/back_button_node.js

@@ -67,11 +67,6 @@ class BackButtonNode extends SAChildNode {
     return false;
   }
 
-  /** @override */
-  isValidAndVisible() {
-    return this.automationNode !== null && !!this.location;
-  }
-
   /** @override */
   onFocus() {
     super.onFocus();

chrome/browser/resources/chromeos/accessibility/switch_access/nodes/group_node.js

@@ -103,7 +103,7 @@ class GroupNode extends SAChildNode {
   isValidAndVisible() {
     for (const child of this.children_) {
       if (child.isValidAndVisible()) {
-        return !!this.location;
+        return super.isValidAndVisible();
       }
     }
     return false;

chrome/browser/resources/chromeos/accessibility/switch_access/nodes/node_wrapper.js

@@ -114,7 +114,8 @@ class NodeWrapper extends SAChildNode {
     if (!this.baseNode_.role) {
       return false;
     }
-    return SwitchAccessPredicate.isVisible(this.baseNode_);
+    return SwitchAccessPredicate.isVisible(this.baseNode_) &&
+        super.isValidAndVisible();
   }
 
   /** @override */

chrome/browser/resources/chromeos/accessibility/switch_access/nodes/switch_access_node.js

@@ -147,9 +147,10 @@ class SAChildNode {
    * Returns whether this node is still both valid and visible onscreen (e.g.
    *    not hidden, not offscreen, not invisible)
    * @return {boolean}
-   * @abstract
    */
-  isValidAndVisible() {}
+  isValidAndVisible() {
+    return !!this.location;
+  }
 
   /**
    * Called when this node becomes the primary highlighted node.