Unknown WebAXRole from Blink should be mapped to a valid chromium AX role.

content/renderer/accessibility/blink_ax_enum_conversion.cc

Index: content/renderer/accessibility/blink_ax_enum_conversion.cc
diff --git a/content/renderer/accessibility/blink_ax_enum_conversion.cc b/content/renderer/accessibility/blink_ax_enum_conversion.cc
index 8d8b3471260990b0c7d9374a2055e22545039110..a9e63476608e818fd96e006d27c72b57d13e0ee5 100644
--- a/content/renderer/accessibility/blink_ax_enum_conversion.cc
+++ b/content/renderer/accessibility/blink_ax_enum_conversion.cc
@@ -333,7 +333,9 @@ ui::AXRole AXRoleFromBlink(blink::WebAXRole role) {
     default:
       // We can't add an assertion here, that prevents us
       // from adding new role enums in Blink.
-      return static_cast<ui::AXRole>(-1);
+      LOG(WARNING) << "Warning: Blink WebAXRole " << role
+                   << " not handled by Chromium yet.";
+      return ui::AX_ROLE_UNKNOWN;
   }
 }