feat: root option in page.accessibility.snapshot() (#4318)

Going from `AXNode` -> `ElementHandle` is turning out to be controversial.

This patch instead adds a way to go from `ElementHandle` -> `AXNode`. If the API looks good, I'll add it into Firefox as well.

References #3641

lib/Accessibility.js

@@ -60,20 +60,36 @@ class Accessibility {
   }
 
   /**
-   * @param {{interestingOnly?: boolean}=} options
+   * @param {{interestingOnly?: boolean, root?: ?Puppeteer.ElementHandle}=} options
    * @return {!Promise<!SerializedAXNode>}
    */
   async snapshot(options = {}) {
-    const {interestingOnly = true} = options;
+    const {
+      interestingOnly = true,
+      root = null,
+    } = options;
     const {nodes} = await this._client.send('Accessibility.getFullAXTree');
-    const root = AXNode.createTree(nodes);
+    let backendNodeId = null;
+    if (root) {
+      const {node} = await this._client.send('DOM.describeNode', {objectId: root._remoteObject.objectId});
+      backendNodeId = node.backendNodeId;
+    }
+    const defaultRoot = AXNode.createTree(nodes);
+    let needle = defaultRoot;
+    if (backendNodeId) {
+      needle = defaultRoot.find(node => node._payload.backendDOMNodeId === backendNodeId);
+      if (!needle)
+        return null;
+    }
     if (!interestingOnly)
-      return serializeTree(root)[0];
+      return serializeTree(needle)[0];
 
     /** @type {!Set<!AXNode>} */
     const interestingNodes = new Set();
-    collectInterestingNodes(interestingNodes, root, false);
-    return serializeTree(root, interestingNodes)[0];
+    collectInterestingNodes(interestingNodes, defaultRoot, false);
+    if (!interestingNodes.has(needle))
+      return null;
+    return serializeTree(needle, interestingNodes)[0];
   }
 }
 
@@ -179,6 +195,21 @@ class AXNode {
     return this._cachedHasFocusableChild;
   }
 
+  /**
+   * @param {function(AXNode):boolean} predicate
+   * @return {?AXNode}
+   */
+  find(predicate) {
+    if (predicate(this))
+      return this;
+    for (const child of this._children) {
+      const result = child.find(predicate);
+      if (result)
+        return result;
+    }
+    return null;
+  }
+
   /**
    * @return {boolean}
    */