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

If not, it leads to a crash later because we don't expect invalied AX roles
in Chromium. We have to tolerate unknown AX roles from Blink because
otherwise there's no way for us to introduce new roles to the Blink enum.

BUG=426608
TBR=aboxhall

Review URL: https://codereview.chromium.org/673143002

Cr-Commit-Position: refs/heads/master@{#301072}

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;
   }
 }