Instrument JSObject::GetHeaderSize to track down Chrome crashes

Bug: chromium:1211264
Change-Id: If47e40e82d79707c7032837f56450790c089cf4b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2976659
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75309}

src/objects/js-objects.cc

@@ -2330,8 +2330,11 @@ int JSObject::GetHeaderSize(InstanceType type,
     case WASM_EXCEPTION_OBJECT_TYPE:
       return WasmExceptionObject::kHeaderSize;
 #endif  // V8_ENABLE_WEBASSEMBLY
-    default:
-      UNREACHABLE();
+    default: {
+      std::stringstream ss;
+      ss << type;
+      FATAL("unexpected instance type: %s\n", ss.str().c_str());
+    }
   }
 }