Add support for retrieving image thumbnails as part of the accessibility tree.

content/renderer/accessibility/blink_ax_tree_source.cc

Index: content/renderer/accessibility/blink_ax_tree_source.cc
diff --git a/content/renderer/accessibility/blink_ax_tree_source.cc b/content/renderer/accessibility/blink_ax_tree_source.cc
index f97a7548cd0737fcbb6b4ea079e5c8393d5ed751..3df538806c4c720443a103f0756b49d51536da24 100644
--- a/content/renderer/accessibility/blink_ax_tree_source.cc
+++ b/content/renderer/accessibility/blink_ax_tree_source.cc
@@ -130,7 +130,6 @@ ScopedFreezeBlinkAXTreeSource::~ScopedFreezeBlinkAXTreeSource() {
 
 BlinkAXTreeSource::BlinkAXTreeSource(RenderFrameImpl* render_frame)
     : render_frame_(render_frame),
-      accessibility_focus_id_(-1),
       frozen_(false) {}
 
 BlinkAXTreeSource::~BlinkAXTreeSource() {
@@ -724,6 +723,11 @@ void BlinkAXTreeSource::SerializeNode(blink::WebAXObject src,
     dst->AddIntAttribute(ui::AX_ATTR_SCROLL_X_MAX, maxScrollOffset.x());
     dst->AddIntAttribute(ui::AX_ATTR_SCROLL_Y_MAX, maxScrollOffset.y());
   }
+
+  if (dst->id == image_data_node_id_) {
+    dst->AddStringAttribute(ui::AX_ATTR_IMAGE_DATA_URL,
+                            src.imageDataUrl(max_image_data_size_).utf8());
+  }
 }
 
 blink::WebDocument BlinkAXTreeSource::GetMainDocument() const {