Add benchmark for LICENSE file size (#23948)

dev/devicelab/lib/tasks/perf_tests.dart

@@ -357,6 +357,7 @@ class CompileTest {
     final _UnzipListEntry isolateSnapshotInstr = fileToMetadata['assets/isolate_snapshot_instr'];
     final _UnzipListEntry vmSnapshotData = fileToMetadata['assets/vm_snapshot_data'];
     final _UnzipListEntry vmSnapshotInstr = fileToMetadata['assets/vm_snapshot_instr'];
+    final _UnzipListEntry license = fileToMetadata['assets/flutter_assets/LICENSE'];
 
     return <String, dynamic>{
       'icudtl_uncompressed_bytes': icudtl.uncompressedSize,
@@ -371,6 +372,8 @@ class CompileTest {
           isolateSnapshotInstr.compressedSize +
           vmSnapshotData.compressedSize +
           vmSnapshotInstr.compressedSize,
+      'license_uncompressed_bytes': license.uncompressedSize,
+      'license_compressed_bytes': license.compressedSize,
     };
   }
 }