[dart2wasm] Ensure --enable-protobuf-mixin-tree-shaker enables --enable-protobuf-tree-shaker

When `--enable-protobuf-mixin-tree-shaker` is enabled then we should
also enable `--enable-protobuf-tree-shaker` internally.

This mirrors the behavior in dart2js which does this in
pkg/compiler/lib/src/options.dart:
```
..enableProtoShaking =
    _hasOption(options, Flags.enableProtoShaking) ||
    _hasOption(options, Flags.enableProtoMixinShaking)
```

Change-Id: I72e9e8951ead747bc8dcb9fb134eb8a6369a3eab
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/490841
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

Author: mkustermann

pkg/dart2wasm/lib/compile.dart

@@ -630,7 +630,9 @@ Future<CompilationResult> _runTfaPhase(
       coreTypes,
       component,
       useRapidTypeAnalysis: true,
-      treeShakeProtobufs: options.translatorOptions.enableProtobufTreeShaker,
+      treeShakeProtobufs:
+          options.translatorOptions.enableProtobufTreeShaker ||
+          options.translatorOptions.enableProtobufMixinTreeShaker,
       treeShakeProtobufMixins:
           options.translatorOptions.enableProtobufMixinTreeShaker,
     );