Add conversions for EmbedObject, Figcaption and Figure

at Web_ax_object_proxy.cc It's for Blink test_runner.

BUG=N/A

Review URL: https://codereview.chromium.org/599363002

Cr-Commit-Position: refs/heads/master@{#296732}

content/shell/renderer/test_runner/web_ax_object_proxy.cc

@@ -78,6 +78,12 @@ std::string RoleToString(blink::WebAXRole role)
       return result.append("Drawer");
     case blink::WebAXRoleEditableText:
       return result.append("EditableText");
+    case blink::WebAXRoleEmbeddedObject:
+      return result.append("EmbeddedObject");
+    case blink::WebAXRoleFigcaption:
+      return result.append("Figcaption");
+    case blink::WebAXRoleFigure:
+      return result.append("Figure");
     case blink::WebAXRoleFooter:
       return result.append("Footer");
     case blink::WebAXRoleForm: