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

ui/accessibility/ax_node_data.cc

Index: ui/accessibility/ax_node_data.cc
diff --git a/ui/accessibility/ax_node_data.cc b/ui/accessibility/ax_node_data.cc
index 66deec361b660a098663da34a9a7490dbd7b4af4..ceb8bea59191bb9ee6350c32b2e6ccc4e6482c43 100644
--- a/ui/accessibility/ax_node_data.cc
+++ b/ui/accessibility/ax_node_data.cc
@@ -616,6 +616,10 @@ std::string AXNodeData::ToString() const {
       case AX_ATTR_HTML_TAG:
         result += " html_tag=" + value;
         break;
+      case AX_ATTR_IMAGE_DATA_URL:
+        result += " image_data_url=(" +
+            IntToString(static_cast<int>(value.size())) + " bytes)";
+        break;
       case AX_ATTR_LANGUAGE:
         result += " language=" + value;
         break;