AX Onion soup: migrate serialization of child's AXTreeID to Blink

Following the lead of crrev.com/c/2140593, this CL moves the logic
that serializes a child frame's AXTreeID from //content/renderer
into Blink itself, and removes WebElement::GetEmbeddingToken() from
Blink's public API.

Bug: 1094150, 1068668
Change-Id: I7ea4def5c65be6135e7e3c956f4a25071831d506
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2302890
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789609}

content/renderer/accessibility/blink_ax_tree_source.cc

@@ -1248,19 +1248,6 @@ void BlinkAXTreeSource::SerializeElementAttributes(WebAXObject src,
       TruncateAndAddStringAttribute(dst, ax::mojom::StringAttribute::kRole,
                                     role);
   }
-
-  // Frames and iframes:
-  // If there are children, the fallback content has been rendered and should
-  // be used instead. For example, the fallback content may be rendered if
-  // there was an error loading an <object>. In that case, only expose the
-  // children. A node should not have both children and a child tree.
-  base::Optional<base::UnguessableToken> child_embedding_token =
-      element.GetEmbeddingToken();
-
-  if (child_embedding_token && !src.ChildCount()) {
-    TruncateAndAddStringAttribute(dst, ax::mojom::StringAttribute::kChildTreeId,
-                                  child_embedding_token->ToString());
-  }
 }
 
 void BlinkAXTreeSource::SerializeHTMLAttributes(WebAXObject src,

third_party/blink/public/web/web_element.h

@@ -33,14 +33,9 @@
 
 #include <vector>
 
-#include "base/optional.h"
 #include "third_party/blink/public/web/web_node.h"
 #include "third_party/skia/include/core/SkBitmap.h"
 
-namespace base {
-class UnguessableToken;
-}
-
 namespace gfx {
 class Size;
 }
@@ -120,11 +115,6 @@ class BLINK_EXPORT WebElement : public WebNode {
   // /public/web interfaces.
   WebString GetComputedValue(const WebString& property_name);
 
-  // Returns the EmbeddingToken associated to this WebElement, or base::nullopt
-  // if there isn't one set. Currently, this can only return actual values for
-  // elements wrapping instances of an blink::HTMLFrameOwnerElement.
-  base::Optional<base::UnguessableToken> GetEmbeddingToken() const;
-
 #if INSIDE_BLINK
   WebElement(Element*);
   WebElement& operator=(Element*);

third_party/blink/renderer/core/exported/web_element.cc

@@ -30,7 +30,6 @@
 
 #include "third_party/blink/public/web/web_element.h"
 
-#include "base/unguessable_token.h"
 #include "third_party/blink/public/platform/web_rect.h"
 #include "third_party/blink/renderer/core/css/css_computed_style_declaration.h"
 #include "third_party/blink/renderer/core/css/css_property_names.h"
@@ -41,7 +40,6 @@
 #include "third_party/blink/renderer/core/html/custom/v0_custom_element.h"
 #include "third_party/blink/renderer/core/html/custom/v0_custom_element_processing_stack.h"
 #include "third_party/blink/renderer/core/html/forms/text_control_element.h"
-#include "third_party/blink/renderer/core/html/html_frame_owner_element.h"
 #include "third_party/blink/renderer/core/html_names.h"
 #include "third_party/blink/renderer/platform/bindings/exception_state.h"
 #include "third_party/blink/renderer/platform/graphics/image.h"
@@ -202,13 +200,6 @@ WebString WebElement::GetComputedValue(const WebString& property_name) {
   return computed_style->GetPropertyCSSValue(property_id)->CssText();
 }
 
-base::Optional<base::UnguessableToken> WebElement::GetEmbeddingToken() const {
-  if (auto* owner = DynamicTo<HTMLFrameOwnerElement>(ConstUnwrap<Element>()))
-    return owner->GetEmbeddingToken();
-
-  return base::nullopt;
-}
-
 WebElement::WebElement(Element* elem) : WebNode(elem) {}
 
 DEFINE_WEB_NODE_TYPE_CASTS(WebElement, IsElementNode())

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

@@ -800,6 +800,25 @@ void AXObject::Serialize(ui::AXNodeData* node_data) {
   if (GetTextAlign() != ax::mojom::blink::TextAlign::kNone) {
     node_data->SetTextAlign(GetTextAlign());
   }
+
+  // If this is an HTMLFrameOwnerElement (such as an iframe), we may need
+  // to embed the ID of the child frame.
+  if (auto* html_frame_owner_element =
+          DynamicTo<HTMLFrameOwnerElement>(GetElement())) {
+    base::Optional<base::UnguessableToken> child_token =
+        html_frame_owner_element->GetEmbeddingToken();
+
+    // Only add the attribute for the embedding token if there aren't children,
+    // because if there are children, the fallback content has been rendered
+    // and should be used instead. For example, the fallback content may be
+    // rendered if there was an error loading an <object>. The AXNodeData
+    // should not have both children and a child tree.
+    if (child_token && !(IsDetached() || ChildCountIncludingIgnored())) {
+      node_data->AddStringAttribute(
+          ax::mojom::blink::StringAttribute::kChildTreeId,
+          child_token->ToString());
+    }
+  }
 }
 
 bool AXObject::IsAXNodeObject() const {