update_engine: Update healthd BusInfo handle logic

Mojo union can now support [Extensible] tag. To support mojo backward
compatibility, we introduced a new field |unmapped_field| in |BusInfo|
to make unrecognized fields deserialize to this field.

Update the handle logic at client side.

BUG=b:232057806
TEST=emerge-volteer update_engine

Cq-Depend: chromium:3835582
Change-Id: I9221b5e4953b0bc9b34cae2770a543b7ec52c980
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/3834753
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
Tested-by: Byron Lee <byronlee@chromium.org>
Commit-Queue: Byron Lee <byronlee@chromium.org>
diff --git a/common/cros_healthd.cc b/common/cros_healthd.cc
index 3a67364..3b7c594 100644
--- a/common/cros_healthd.cc
+++ b/common/cros_healthd.cc
@@ -343,6 +343,11 @@
         case chromeos::cros_healthd::mojom::BusInfo::Tag::kThunderboltBusInfo: {
           break;
         }
+        case chromeos::cros_healthd::mojom::BusInfo::Tag::kUnmappedField: {
+          LOG(ERROR) << "Get unmapped Mojo fields by retrieving bus info from "
+                        "cros_healthd";
+          break;
+        }
       }
     }
   }